MLB Prop Bet Glossary: Every Term a UK Punter Needs to Know
Available in US
A Shared Vocabulary Makes Every Other Guide on This Site Easier to Follow
I spent my first month reading American betting content with a browser tab permanently open to Google, searching terms I had never encountered. “What is a moneyline?” “What does the run line mean?” “Why do Americans write odds with a plus sign?” If you have asked yourself any of these questions, this glossary exists for you. Baseball betting has its own language — a blend of American sports-betting jargon and sabermetric terminology that can feel impenetrable to a UK punter whose frame of reference is decimal odds and football accumulators.
The terms below are organised alphabetically and written for a British audience. Where American and UK terminology overlap or differ, I have noted the distinction. This is a reference page — bookmark it, return to it, and use it as a companion whenever another article on this site introduces a term that does not immediately click.
Terms A-M
Action: A bet that is live and has not been cancelled or voided. If you have “action” on a game, you have money riding on it.
Alternate line: A prop line set at a different number from the primary market, with adjusted odds. If the main strikeout line is 5.5, an alternate line might be 4.5 (lower odds for the over) or 6.5 (higher odds for the over).
BABIP (Batting Average on Balls in Play): The percentage of balls put in play that fall for hits, excluding home runs. League average sits around .300. Extreme BABIP values — above .350 or below .250 — tend to regress toward the mean, which affects hits and total bases props.
Barrel: A batted ball with a combination of exit velocity and launch angle that historically produces a batting average above .500 and a slugging percentage above 1.500. Barrel rate is a key metric for home run prop evaluation.
Bet builder: The UK term for a same-game parlay. Multiple selections from a single game combined into one bet with enhanced odds. Known as SGP (same-game parlay) in American markets.
Bullpen: The group of relief pitchers available to enter the game after the starting pitcher is removed. Bullpen workload and fatigue directly affect late-game prop outcomes.
Chase rate: The percentage of pitches thrown outside the strike zone that a batter swings at. High chase rates indicate aggressive, swing-happy hitters who are more likely to strike out.
Closing line: The final odds available on a prop just before the game starts. Beating the closing line consistently — placing bets at better odds than the final price — is a strong indicator of long-term betting skill.
Decimal odds: The standard odds format used by UK sportsbooks. The number represents the total return per pound wagered, including the original stake. Odds of 1.90 return 1.90 pounds on a 1-pound bet (0.90 profit). This is the format UK punters should use by default.
Edge: The difference between your estimated probability of an outcome and the implied probability of the odds. A positive edge means you believe the bet will win more often than the price suggests.
Exit velocity: The speed of the ball off the bat, measured in miles per hour. Higher exit velocity correlates with more extra-base hits and home runs. A key metric for total bases and home run props.
Expected value (EV): The average amount you would win or lose per bet if you placed the same bet at the same odds an infinite number of times. Positive EV (+EV) bets are profitable in the long run; negative EV (-EV) bets are not.
FIP (Fielding Independent Pitching): A pitcher metric that evaluates performance based solely on strikeouts, walks, hit batsmen and home runs — outcomes the pitcher controls directly. More stable and predictive than ERA for prop evaluation.
Fractional odds: The traditional UK odds format (e.g., 9/10, 5/4). Less common on modern sportsbook apps but still used by some platforms. 9/10 means a 10-pound bet returns 9 pounds profit plus the original stake.
Handle: The total amount of money wagered on a market. High-handle markets are more liquid and tend to have tighter odds.
Hold rate: The percentage of total handle that the sportsbook retains as profit. On single bets, the hold rate is typically 4-5%. On parlays and same-game parlays, it can reach 20-35%.
Implied probability: The probability of an outcome as suggested by the odds. Calculated as 1 divided by the decimal odds. Odds of 2.00 imply a 50% probability; odds of 1.50 imply 66.7%.
Juice: Another term for the vig (vigorish). The sportsbook’s built-in margin on a bet. Also called “the vig” or “the overround” in UK terminology.
Launch angle: The vertical angle at which the ball leaves the bat. Combined with exit velocity, it predicts the type of batted-ball outcome. Home runs typically require a launch angle between 25 and 35 degrees at high exit velocity.
Line: The number set by the sportsbook for a prop market. “The line is 5.5 strikeouts” means the prop is set at that threshold, with over and under options.
Moneyline: The American odds format. Positive numbers (+150) indicate the profit on a 100-unit stake; negative numbers (-130) indicate the stake needed to win 100 units. UK bettors should convert to decimal for easier comparison. +150 = 2.50 decimal. -130 = 1.77 decimal.
Terms N-Z
NRFI (No Run First Inning): A bet that neither team will score a run in the first inning. The opposite is YRFI (Yes Run First Inning).
Over/under (O/U): A bet on whether a statistical total will be higher (over) or lower (under) than the posted line. The most common prop bet format.
Park factor: A ratio that compares offensive production at a specific ballpark to the league average. A factor of 110 means the park inflates offence by 10% compared to neutral.
Parlay: A single bet combining multiple selections. All selections must win for the parlay to pay out. Known as an “accumulator” or “acca” in UK betting terminology. Carries a higher house edge than single bets.
Platoon split: The difference in a player’s performance against left-handed versus right-handed opponents. Most hitters perform better against opposite-handed pitchers.
Pop time: The time from a pitch hitting the catcher’s mitt to the throw arriving at second base. A key metric for stolen base prop evaluation. Elite catchers register pop times below 1.90 seconds.
Prop bet (proposition bet): A wager on a specific event or statistical outcome within a game, rather than on the game’s final result. Player props focus on individual performance; game props cover team-level events.
Push: When the outcome exactly matches the line (e.g., the pitcher records exactly 6 strikeouts on a 6.0 line). The bet is voided and the stake is returned. Half-point lines (5.5, 6.5) eliminate the possibility of a push.
Run line: The MLB equivalent of a points spread. The standard run line is -1.5/+1.5, meaning the favourite must win by two or more runs to cover. Affects prop evaluation through its influence on game flow and late-game substitution patterns.
Same-game parlay (SGP): A parlay where all legs come from the same game. Called a “bet builder” on most UK sportsbooks. Carries a significantly higher hold rate than standard parlays.
Sharp: A professional or highly skilled bettor whose wagers move the line. Sharp action on a prop indicates informed money.
Sprint speed: A player’s top running speed measured in feet per second. Used to evaluate stolen base prop potential. Elite sprint speed is above 30 ft/s.
Statcast: MLB’s ball-tracking technology that measures pitch velocity, spin rate, exit velocity, launch angle, sprint speed and defensive positioning. The data is published free on Baseball Savant.
Total bases: The sum of bases accumulated through hits. A single = 1, double = 2, triple = 3, home run = 4. Walks, hit-by-pitches and fielding errors do not count toward total bases.
Vig (vigorish): The sportsbook’s margin on a bet. On a standard -110/-110 market (1.91/1.91 decimal), the vig is approximately 4.5%. Also called “juice” in American terminology or “overround” in British terminology.
Whiff rate: The percentage of swings that result in a miss. A pitcher’s whiff rate is the most predictive metric for future strikeout performance. League average sits around 24-25%.
wRC+ (Weighted Runs Created Plus): An all-encompassing offensive metric scaled so that 100 is league average. A wRC+ of 130 means the hitter produces 30% more offensive value than the average player. Useful for comparing hitters across different parks and eras.
xERA (Expected ERA): A pitcher’s estimated ERA based on the quality of contact allowed, independent of defensive support and sequencing luck. When actual ERA diverges significantly from xERA, regression is likely — a key signal for earned-run prop evaluation.
xSLG (Expected Slugging): A hitter’s estimated slugging percentage based on exit velocity and launch angle data. When actual slugging lags behind xSLG, the hitter is likely to see improved results going forward, which informs total bases and home run prop analysis.
YRFI (Yes Run First Inning): A bet that at least one team will score a run in the first inning. The opposite of NRFI. Value often appears when two back-end starters with weak first-inning splits face aggressive top-of-order lineups.
This glossary covers the terms that appear most frequently across the analysis on this site. If you encounter a term in another article that is not listed here, the context should make it clear — but if it does not, the data sources covered in our guide to UK sportsbook navigation will fill any remaining gaps.
What is the difference between a prop bet and a futures bet in MLB?
A prop bet is a wager on a specific outcome within a single game — such as a pitcher’s strikeout total or whether a hitter will record a home run. A futures bet is a long-term wager on a season-level outcome — such as which team will win the World Series, which player will win the MVP award, or a team’s regular-season win total. Props resolve in one game; futures resolve over weeks or months.
What does ‘vig’ mean and why does it matter for prop bettors?
The vig — short for vigorish, also called juice or overround — is the sportsbook’s built-in margin on every bet. On a standard prop market, both sides are priced below true even odds, so the combined implied probabilities exceed 100%. The difference is the vig. It matters because a higher vig means you need a higher win rate to break even. On prop markets, the vig is typically 4-8%, compared to 3-5% on main match-result lines.
Written by the editors at mlb bet Props.
