Odds
Odds tell you how likely an outcome is and how much a winning bet pays back relative to your stake.
Definition
In gambling, "odds" carries two linked meanings: the true (mathematical) odds that describe an event's probability, and the odds a bookmaker or operator actually offers, which are the price paid on a winning bet. Offered odds are quoted in different formats — decimal (e.g. 2.00), fractional (e.g. 1/1), or American/moneyline (e.g. +100) — that all convert to the same implied probability. Operators build a margin into their prices, so the implied probabilities across all outcomes of an event add up to more than 100%; that extra slice, called the overround or vigorish, is the operator's expected edge — the sportsbook analogue of a casino's house edge.
Worked example
Take a near-even market such as a two-player tennis match. If both players were genuinely 50/50, fair decimal odds would be 2.00 on each side (implied probability = 1 / 2.00 = 50%). A bookmaker instead offers 1.91 on each side. Each price implies 1 / 1.91 = 52.36%, so the two outcomes together imply 104.72% rather than 100%. That surplus of 4.72% is the overround — the operator's built-in margin. The same 1.91 price also equals roughly fractional 10/11 and American -110; format changes, but the underlying implied probability does not.
Why it matters
Reading odds is the foundation of understanding any betting market, because a single number encodes both an outcome's probability and its payout. Being able to convert odds into implied probability lets a learner see the operator's margin hidden inside a price and compare which operator is quoting the sharper (lower-margin) line. For professionals, setting odds and managing the overround is central to how a sportsbook prices risk and generates revenue, just as RTP and house edge govern casino game economics. Recognizing that offered odds already embed a margin also makes clear why the pricing structure is designed to favor the operator over time.
Related
Note: The core definition is stable, but odds formats are regional conventions — decimal is common in Europe, Australia, and Canada; fractional in the UK and Ireland; American/moneyline in the US. The specific margin figures (e.g. the ~4.7% overround shown) are illustrative only: real-world margins vary widely by operator, sport, and market and can range from under 2% to over 10%.