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The Legends Deck glossary defines every MLB Statcast and baseball stat used in the game — each term links to a full explainer with the current 2026 leaders.
136 terms across 7 categories. Browse the blog for guides and deep dives.
A slash line is a three-number shorthand displaying a batter's Batting Average, On-Base Percentage, and Slugging Percentage separated by slashes — for example, .300/.380/.520 — giving a compact portrait of contact, discipline, and power.
Attack Angle is the vertical angle of the bat's path at the moment of contact, measured by Statcast bat tracking, indicating whether a hitter is swinging up, down, or level through the ball.
Barrel rate is the percentage of a hitter's batted balls classified as barrels — contact of at least 98 mph exit velocity in a launch-angle window that has historically produced a .500+ batting average and 1.500+ slugging — making it the single best Statcast predictor of sustainable power.
Bat speed is the velocity of the bat's sweet spot at contact, measured in miles per hour by Statcast's Hawkeye system, with an MLB league average near 71 mph and elite hitters exceeding 75 mph.
Chase rate is the percentage of pitches outside the strike zone that a batter swings at, measuring plate discipline where a lower number indicates a more selective hitter.
Exit velocity is the speed of the baseball as it leaves the bat on a batted ball, measured in miles per hour by Statcast's Hawk-Eye optical tracking in every MLB stadium; it is the cleanest single measure of raw contact quality.
Fly ball rate (FB%) is the percentage of a hitter's or pitcher's batted balls that are classified as fly balls, a core component of every batted-ball profile.
Hard-hit rate is the percentage of a hitter's batted balls struck at 95 mph or harder off the bat, a Statcast quality-of-contact measure that stabilizes quickly and correlates strongly with power and expected slugging.
ISO (Isolated Power) measures a hitter's raw extra-base power by subtracting batting average from slugging percentage, isolating only doubles, triples, and home runs.
Launch angle is the vertical angle at which the ball leaves the bat after contact, measured in degrees by Statcast's optical tracking.
On-Base Percentage (OBP) is the rate at which a hitter reaches base via hit, walk, or hit-by-pitch, calculated as (H + BB + HBP) ÷ (AB + BB + HBP + SF).
OPS (On-base Plus Slugging) is the sum of a hitter's on-base percentage and slugging percentage, used as a quick single-number measure of total offensive production.
OPS+ is on-base plus slugging percentage adjusted for park and league context, scaled so that 100 equals league average — a 150 OPS+ means a hitter was 50% better than league average at producing offense.
Pull rate is the percentage of a hitter's batted balls hit to the side of the field they swing toward — left field for right-handed batters, right field for left-handed batters.
RBI (runs batted in) credits a batter for each runner who scores as a direct result of his plate appearance, excluding runs that score on errors or grounded-into double plays.
Slugging percentage (SLG) is a hitter's total bases divided by at-bats, weighting singles as 1, doubles as 2, triples as 3, and home runs as 4 to measure per-at-bat power output.
Squared-up rate is the percentage of swings on which a hitter transfers at least 80% of the maximum possible exit velocity to the ball, given their bat speed and the incoming pitch speed.
Sweet Spot Percentage is the share of a hitter's batted balls struck with a launch angle between 8 and 32 degrees — the optimal range for producing hits and extra-base damage.
Swing length is the total distance, measured in feet, that the head of the bat travels from the start of a hitter's swing through contact, tracked by Statcast's bat-tracking system.
The Mendoza Line is the .200 batting-average threshold that separates barely-employable major-league hitters from those almost certain to lose their job.
The three true outcomes are strikeouts, walks, and home runs — plate appearances whose outcomes are decided entirely by the pitcher and hitter with no involvement from fielders.
xBA (expected batting average) is a Statcast metric that estimates what a hitter's batting average should be based on the exit velocity, launch angle, and sprint speed of each batted ball, stripping out luck and defense.
Expected Slugging Percentage (xSLG) is a Statcast metric that estimates a hitter's slugging percentage from exit velocity, launch angle, and sprint speed, stripping out the effects of defense, ballpark, and luck.
A changeup is an off-speed pitch thrown with fastball arm action but at reduced velocity, designed to disrupt a hitter's timing through deception rather than break.
A curveball is a slow breaking pitch thrown with heavy topspin that drops sharply as it crosses the plate, typically 70-82 mph in MLB.
A cutter is a fastball variant — typically 2-5 mph slower than a four-seamer — that breaks late toward the pitcher's glove side, splitting the difference between a fastball and a slider.
A four-seam fastball is baseball's most common pitch — gripped across the horseshoe seams to generate maximum backspin and near-zero horizontal movement, producing the highest velocity and the most carry through the strike zone of any pitch type.
A hold is a relief-pitching stat awarded when a pitcher enters in a save situation, records at least one out, and leaves the game with the lead still intact for a teammate to finish.
A knuckle-curve is a curveball thrown with one or two fingers spiked (knuckled) against the ball, producing tight, late vertical break with curveball-level spin rates.
A knuckleball is a pitch thrown with minimal spin — typically under 1,500 RPM — that flutters unpredictably toward the plate due to asymmetric airflow across its seams, making it difficult for hitters and catchers alike to track.
A quality start is any starting-pitcher outing of at least six innings with three or fewer earned runs allowed — a binary credit for keeping the team in the game.
A save is a statistic credited to a relief pitcher who finishes a game won by their team while meeting MLB Rule 9.19 criteria — generally entering in a tight spot and protecting a lead of three runs or fewer.
A sinker is a fastball variant with arm-side run and downward movement that induces ground balls, thrown 88-96 mph with a spin axis tilted toward the throwing arm.
A slider is a breaking pitch thrown between 78 and 92 mph that moves laterally and downward, sitting between a fastball and a curveball in velocity and break.
A slurve is a breaking pitch that combines the horizontal sweep of a slider with the downward bite of a curveball, traveling 78–84 mph with a diagonal 11-to-5 shape.
A splitter is an off-speed pitch thrown with the index and middle fingers split wide on the ball, producing sharp late downward movement at near-fastball velocity.
A sweeper is a breaking pitch thrown in the low-to-mid 80s with exaggerated horizontal break—often 15 to 20 inches—and minimal vertical drop, classified by Statcast as a distinct pitch from the traditional slider.
A two-seam fastball is a fastball gripped along the narrow parallel seams of the baseball, producing arm-side run and natural downward movement that generates weak contact and groundballs.
An immaculate inning is a half-inning in which one pitcher strikes out all three batters on exactly nine pitches — every pitch a strike, no contact, no balls.
An opener is a relief pitcher used to start a game and work only the first inning before handing off to a bulk reliever, designed to neutralize the top of the order and dodge the times-through-the-order penalty.
Arm angle is the angle in degrees between a pitcher's shoulders and the line from shoulder to ball release, where 0° is sidearm, 90° is straight overhead, and negative values are submarine.
Bauer Units (BU) measure fastball spin rate divided by velocity, expressing how much spin a pitcher generates per mile per hour.
CSW (Called Strikes plus Whiffs) Rate is the percentage of a pitcher's total pitches that result in either a called strike or a swinging strike, capturing both command and stuff in one number.
DRA (Deserved Run Average) is Baseball Prospectus's run-prevention metric that estimates how many runs a pitcher deserved to allow per nine innings after stripping out the influence of defense, framing, park, and opponent quality.
ERA (Earned Run Average) is the number of earned runs a pitcher allows per nine innings, calculated as (Earned Runs × 9) ÷ Innings Pitched.
FIP (Fielding Independent Pitching) is a pitcher's ERA estimator built only from strikeouts, walks, hit-by-pitches, and home runs — outcomes the pitcher controls directly, independent of the defense behind him.
Game Score is a single-number metric created by Bill James that grades an individual starting pitcher's performance in one game, where 50 is average and 100+ is historically elite.
Ground Ball Rate (GB%) is the percentage of balls put in play — excluding home runs — that travel along the ground, and it is the primary batted-ball metric for gauging a pitcher's ability to suppress extra-base contact and generate double plays.
Horizontal Approach Angle (HAA) is the side-to-side angle in degrees at which a pitch crosses the front of home plate, measuring how sharply a pitch cuts across the strike zone from the hitter's view.
Horizontal break measures how many inches a pitch moves side-to-side relative to a spinless trajectory, capturing the arm-side run or glove-side sweep a hitter has to track from release to the plate.
HR/9 (home runs allowed per nine innings) is a rate stat that measures how often a pitcher gives up home runs, normalized to a full game's workload — calculated as (HR ÷ IP) × 9.
Induced Vertical Break (IVB) is the amount of vertical movement on a pitch caused by spin and aerodynamics after the effect of gravity is subtracted, measured in inches by Statcast.
K-BB% (strikeout rate minus walk rate) measures a pitcher's ability to generate strikeouts while limiting walks, making it one of the most stable and predictive single-number indicators of pitching quality.
K/9 is a rate stat that expresses how many batters a pitcher strikes out per nine innings of work, providing a workload-normalized view of swing-and-miss production.
Leverage Index (LI) quantifies the importance of any plate appearance by measuring how much the game's win probability could swing on its outcome relative to an average situation, where 1.0 is league-average leverage.
LOB% (left-on-base percentage, also called strand rate) measures the share of baserunners a pitcher prevents from scoring; league average sits around 72-73%.
Location+ is a Stuff+ family pitching metric that grades how well a pitcher locates each pitch given the count, batter handedness, and pitch type, scaled so 100 is league average and 10 points equals one standard deviation.
Perceived velocity is the effective speed of a pitch as experienced by the hitter, adjusting raw release velocity for the pitcher's extension toward home plate.
Pitch tunneling is the technique of throwing different pitches along the same visible path for as long as possible, so the hitter can't distinguish them before the swing decision point roughly 23–25 feet from home plate.
Pitcher extension is the distance in feet from the front of the pitching rubber to where the pitcher releases the ball, measured by Statcast — longer extension makes a fastball play faster than its radar-gun reading.
Pitching+ is a FanGraphs model that combines Stuff+ (pitch shape quality) and Location+ (command quality) into a single 100-scaled metric, where 100 is league average and every point above represents roughly 1% better expected run prevention.
Release point is the three-dimensional coordinate where a pitcher's hand lets go of the baseball, captured in feet by Statcast and used to evaluate arm slot, command consistency, and pitch deception.
Seam-shifted wake is non-Magnus pitch movement created when the orientation of a baseball's raised seams steers airflow, producing break that pure spin physics can't explain.
Spin axis is the orientation of a baseball's rotational axis as it travels toward home plate, expressed as a clock-face direction that determines whether a pitch carries, sinks, cuts, or sweeps.
Spin efficiency is the percentage of a pitch's total spin that actually creates movement — the rest is gyrospin that contributes nothing to break.
Spin rate is the number of times a pitched baseball rotates around its axis per minute (RPM), measured by Statcast, and is a primary driver of pitch movement and swing-and-miss rates.
Stuff+ is a pitch-quality model that rates the physical nastiness of a pitch — velocity, spin, and movement — on a scale where 100 is MLB average and every 10 points equals one standard deviation.
Swinging strike rate (SwStr%) is the percentage of a pitcher's total pitches that result in a swing-and-miss, measuring raw bat-missing ability across every pitch thrown.
The eephus is an ultra-slow, high-arcing lob pitch — typically 45 to 65 mph — designed to disrupt a hitter's timing by appearing 30-plus mph slower than a pitcher's fastball.
A screwball is a rare breaking pitch that moves opposite of a curveball — fading and dropping toward the pitcher's arm side — created by pronating the wrist outward at release rather than supinating it.
Vertical Approach Angle (VAA) is the downward angle at which a pitch crosses home plate, measured in degrees, and is the key Statcast metric for explaining why some fastballs miss bats at the top of the zone.
Whiff rate is the percentage of a batter's swings that miss the ball, calculated as swings-and-misses divided by total swings; it is the cleanest single measure of a pitch's bat-missing ability.
WHIP (Walks plus Hits per Inning Pitched) measures how many baserunners a pitcher allows per inning, calculated as walks plus hits divided by innings pitched.
xERA is Statcast's expected ERA estimator, built from a pitcher's quality-of-contact data, strikeouts, and walks to predict run prevention skill independent of defense and sequencing luck.
xFIP (Expected Fielding Independent Pitching) is a defense-neutral ERA estimator that normalizes a pitcher's home run rate to league average, isolating only the outcomes pitchers control: strikeouts, walks, and fly balls.
Arm strength is the Statcast metric measuring a fielder's maximum throwing velocity in miles per hour, used to grade outfield arms and infielder throwing tools.
Catch Probability is a Statcast metric that estimates the likelihood — expressed as a percentage from 0 to 100 — that a major-league outfielder will catch a specific fly ball or line drive, based solely on the distance to the landing zone and the time available to get there.
Catcher framing is the skill of converting borderline pitches into called strikes through pitch receiving, measured by Statcast as the difference between actual and expected called-strike rate in the shadow zone.
Defensive Runs Saved (DRS) is a comprehensive fielding metric that estimates how many runs a defender saves or costs his team relative to an average player at his position.
OAA (Outs Above Average) is Statcast's primary defensive metric, measuring how many outs a fielder converts above or below what an average fielder would produce given identical opportunities.
Pop time is the number of seconds from the instant a pitch hits the catcher's mitt to the instant the ball reaches the fielder's glove at the base on a throw to second or third, measuring a catcher's total throw-down quickness.
Sprint speed is a player's average peak running speed in feet per second, measured by Statcast across their fastest one-second window on qualified runs; it is the cleanest single measure of raw athletic speed in baseball.
The shift is a defensive alignment that positions three or more infielders on one side of second base to take away pull-heavy hitters' most likely ground-ball zones — a strategy banned in MLB starting with the 2023 season.
UZR (Ultimate Zone Rating) measures a fielder's total defensive value in runs above or below an average player at the same position, combining range, errors, double plays, and outfield arm strength.
BABIP (Batting Average on Balls in Play) measures how often a hitter reaches base on balls put in play, excluding home runs and strikeouts, with league average hovering near .300.
BsR (Base Running Runs) is FanGraphs' all-in-one baserunning metric that measures how many runs a player adds or costs his team on the bases — including stolen bases, extra-base advancement on hits, and double-play avoidance — relative to a league-average runner.
Clutch is a FanGraphs metric that measures how much better or worse a player performed in high-leverage situations compared to his own context-neutral production.
ERA+ (Adjusted ERA Plus) is a park- and league-adjusted version of ERA where 100 equals league average and every point above 100 means the pitcher allowed 1% fewer runs than a typical pitcher in the same conditions.
K% (strikeout rate) is the share of plate appearances ending in a strikeout, calculated as strikeouts divided by plate appearances, and used to measure both hitter contact ability and pitcher dominance.
Pythagorean Winning Percentage estimates a team's expected winning percentage from runs scored and runs allowed using the formula RS² / (RS² + RA²), revealing teams that are over- or under-performing their underlying run differential.
Run Value is a Statcast metric that measures how many runs a single pitch, plate appearance, or season's worth of events added or subtracted relative to league average, calculated from the run expectancy matrix.
SIERA (Skill-Interactive ERA) is an ERA estimator that predicts a pitcher's true run-prevention skill from strikeouts, walks, and batted-ball mix while accounting for how those skills interact.
Walk rate (BB%) is the percentage of a hitter's or pitcher's plate appearances that end in a base on balls, measuring plate discipline for hitters and command for pitchers.
WAR (Wins Above Replacement) measures how many wins a player contributes above what a freely available replacement-level player would provide, combining offense, defense, baserunning, and pitching into a single number.
Platoon splits measure how a hitter or pitcher performs against same-handed versus opposite-handed opponents — the foundation of lineup and bullpen strategy.
wOBA (weighted On-Base Average) is a rate stat that credits each offensive event — walk, single, double, triple, home run — by its actual run value, then scales the result to the familiar OBP range.
Win Probability Added (WPA) measures how much each plate appearance changes a team's probability of winning, summed across a full season, to quantify a player's clutch contribution to actual wins.
wRC+ (Weighted Runs Created Plus) is a park- and league-adjusted measure of total offensive value where 100 is league average and every point above or below equals one percent better or worse than average.
xwOBA (expected weighted On-Base Average) is Statcast's quality-of-contact metric that estimates what a hitter's wOBA should be based purely on exit velocity, launch angle, and (for speed-dependent balls) sprint speed.
ZiPS is a player projection system created by Dan Szymborski that forecasts MLB hitter and pitcher performance using weighted historical data, similarity scores, and empirical aging curves.
A balk is an illegal pitching motion with runners on base that the umpire judges to deceive the runner — the ball is dead and every runner advances one base.
A fielder's choice is when a batter reaches base only because the defense chose to put out a different baserunner instead of throwing him out, crediting no hit.
A passed ball is a pitch that the catcher should have caught or controlled with ordinary effort but fails to, allowing a baserunner to advance — charged to the catcher, not the pitcher.
A perfect game is a complete game in which a single pitcher (or pitchers) retires all 27 batters faced, allowing no hits, no walks, no hit-by-pitches, and no baserunners of any kind.
A swing-off in baseball is the All-Star Game tiebreaker that replaces extra innings with a Home Run Derby-style mini-contest: each side picks three hitters who take three swings apiece, and the team that hits the most home runs wins.
A walk-off is a play in which the home team scores the winning run in the bottom of the ninth inning or later, ending the game instantly because the visiting team has no further at-bat.
A wild pitch is a pitch so high, wide, or low that the catcher cannot control it with ordinary effort, allowing a baserunner to advance, charged to the pitcher rather than the catcher.
Hitting for the cycle is when a single batter records a single, double, triple, and home run in the same game — one of baseball's rarest individual feats, rarer than a no-hitter.
The designated hitter (DH) is a player who bats in place of the pitcher every time the pitcher's spot comes up, without ever playing the field.
The ghost runner is an automatic runner placed on second base at the start of each extra-inning half-inning in MLB regular-season games, introduced in 2020 and made permanent in 2023.
The infield fly rule automatically retires the batter on a fair pop-up an infielder can catch with ordinary effort when there are runners on first and second (or bases loaded) with fewer than two outs — preventing fielders from intentionally dropping the ball for cheap double plays.
A team's magic number is the combined total of its own remaining wins plus its closest pursuer's losses required to mathematically clinch a playoff spot or division title — when it hits zero, the spot is secured.
The pitch clock is an MLB rule introduced in 2023 that limits time between pitches—15 seconds with bases empty, 18 with runners on—and cut average game time by 24 minutes in its first season.
The strike zone is the rulebook three-dimensional space over home plate, from the midpoint between the batter's shoulders and waist down to the hollow beneath the kneecap, in which a pitch that is not swung at is called a strike.
A club option is a contract provision that gives a team the unilateral right to extend a player's contract for one or more additional years at a pre-specified salary, in exchange for paying a smaller buyout if the team declines.
A minor league option lets a team freely send a player on the 40-man roster to the minors during a season; each player gets three option years, with a possible fourth under the 2022 CBA.
A no-trade clause is a contract provision giving an MLB player the right to veto any trade — either to all 29 other teams (full NTC) or to a specified list (limited NTC).
A non-tender is when an MLB team declines to offer a contract to one of its arbitration-eligible players by the annual tender deadline, instantly making the player a free agent.
Deferred money is salary a player agrees to be paid in future years — often long after the contract ends — lowering the deal's present-day cost and luxury-tax hit.
Designated for Assignment (DFA) is an MLB roster move that removes a player from the 40-man roster, opening a seven-day window for the team to trade, waive, outright, or release him.
MLB salary arbitration is the contract process for players with 3-6 years of service time where, if the player and team can't agree on a salary, an independent panel must choose between the two filed figures with no compromise allowed.
Service time is the count of days a player spends on an MLB active roster or injured list, where 172 days equals one full service year and accrual unlocks arbitration at three years and free agency at six.
Super Two is the designation given to the top 22% of MLB players with between 2 and 3 years of service time, granting them an extra year of arbitration eligibility before free agency.
The 40-man roster is the group of players under MLB contract who occupy a team's 40 player slots, are protected from the Rule 5 Draft, and form the pool from which the 26-man active roster is drawn.
The injured list (IL) is MLB's roster mechanism for sidelining medically restricted players, freeing the 26-man spot for replacements while preserving the player's service time and full salary.
The luxury tax, formally MLB's Competitive Balance Tax, is a graduated penalty paid by teams whose payroll exceeds annual thresholds set in the Collective Bargaining Agreement, with rates that escalate by tier and by repeat-offender status.
The qualifying offer is a one-year guaranteed contract a team can extend to its own impending free agent at the average of the top 125 MLB salaries — declining it attaches draft-pick compensation that depresses the player's market value.
The Rule 5 Draft is an annual MLB event held during the Winter Meetings that lets teams select unprotected minor-league players from other organizations, with strict roster rules designed to prevent talent hoarding.
A five-tool player is a position player who grades plus-or-better at all five core baseball skills: hitting for average, hitting for power, running speed, fielding ability, and throwing arm.
Fielding percentage measures the rate at which a fielder successfully handles the chances he reaches, calculated as putouts plus assists divided by total chances.
Park factor is a ratio that captures how much a specific ballpark inflates or depresses offensive events (runs, home runs, hits) relative to a neutral environment.
PitchCom is the MLB-approved encrypted wearable that lets catchers send pitch type and location calls electronically to pitchers and fielders, replacing traditional finger signs.
Statcast is MLB's radar-and-camera tracking system that measures every pitch, swing, and sprint at the major-league level.
The 20-80 scouting scale is the industry-standard grading system pro scouts use to rate baseball tools, where 50 is MLB-average, 80 is elite, and 20 is well below replacement level.
A forkball is an off-speed pitch thrown with the index and middle fingers spread wide around the ball, producing low spin and a sharp downward tumble that mimics a fastball before falling out of the strike zone.
Tommy John surgery reconstructs a torn ulnar collateral ligament in the elbow with a tendon graft, with a 12-18 month return-to-pitch timeline and an 80-85% success rate.