Reference
Glossary
The recurring vocabulary of casino mathematics, translated into plain English.
House edge
The casino's built-in mathematical advantage on a given game.
The house edge is the percentage of each bet the casino expects to keep on average over the long run. On European roulette it is 2.70%; on American roulette it is 5.26%. It is a property of the game's rules, not of any single session.
Expected value (EV)
The average outcome of a bet if it were repeated many times.
Expected value multiplies each possible outcome by its probability and sums them. In a game with a house edge, EV is negative — every bet has an average cost even though any single bet can win or lose.
Return to player (RTP)
The mirror image of house edge, quoted from the player's side.
RTP is 100% minus the house edge. A slot advertised at 96% RTP has a 4% house edge. RTP is a long-run average across millions of spins, not a guarantee for any single session.
Variance
How widely individual results scatter around the average.
Variance describes short-term swings. High-variance games produce rare, large wins between long stretches of losses; low-variance games produce frequent, small results. Variance changes how a session feels — it does not change the house edge.
Standard deviation
A single number that summarises the size of typical swings.
Standard deviation is the square root of variance. It gives a sense of how far a typical result sits from the average, which is why two games with the same RTP can feel completely different to play.
Law of large numbers
As samples grow, averages converge toward the true probability.
Twenty coin flips can look wildly unbalanced. Five thousand almost never do. The law of large numbers is the reason the casino's edge is invisible in the short run and inevitable in the long run.
Independent events
Outcomes that do not influence one another.
Each spin of a roulette wheel or slot is independent of the last. Previous results carry no information about the next one, no matter how strong the pattern appears.
Gambler's fallacy
The belief that past results shift the odds of future ones.
After five reds on roulette, black is no more likely than before. The wheel has no memory. Treating independent events as if they balance out in the short run is one of the most common cognitive errors in gambling.
Hot hand fallacy
The belief that a streak of wins signals an ongoing advantage.
Winning streaks feel meaningful but are a natural product of randomness. In fixed-odds games no player is ever 'due' or 'hot' — the probabilities reset every bet.
Wagering requirement
A multiplier attached to a bonus that sets required turnover.
A £20 bonus at 40× wagering requires £800 of turnover before the balance can be withdrawn. The multiplier does not slow a withdrawal — it decides how many bets the house edge is applied to.
Turnover
The total amount staked across a period, not the amount lost.
A player who stakes £1 a hundred times has £100 of turnover, even if their balance barely moves. Turnover — not net deposit — is what house edge acts on.
Contribution rate
How much a given game counts toward wagering requirements.
Slots typically contribute 100% to wagering; table games often contribute 10% or 0%. Lower contribution rates quietly multiply the turnover needed to clear a bonus.
Volatility
Informal term used interchangeably with variance, especially on slots.
A 'high volatility' slot pays less often but in larger amounts. A 'low volatility' slot pays frequently in smaller amounts. Same RTP, very different session experience.
Bankroll
The total amount of money set aside for play.
Bankroll is the reference point for measuring session outcomes. It is the number the long-run mathematics of expected value quietly acts on.
Convergence
The tendency of averages to approach their true value over time.
Short-run results scatter; long-run averages converge. Convergence is the mechanism that turns a small per-bet house edge into a predictable outcome for the casino across millions of bets.