Lay Bet
Betting that an outcome will NOT happen, effectively acting as the bookmaker by accepting someone else's backing stake.
Definition
To lay a bet is to bet against an outcome: you win the backer's stake if the outcome does not occur, but you must cover their winnings, your liability, if it does. Lay betting is the defining feature of a betting exchange and mirrors what a traditional bookmaker does. Your liability equals (odds - 1) x the backer's stake, which for long odds can be many times the amount you stand to win, so laying long-odds outcomes carries large downside relative to the reward.
Worked example
You lay Team A at 3.0 against a $100 backer stake. If Team A does not win, you keep the $100. If Team A wins, you pay the backer (3.0 - 1) x $100 = $200 liability. So you risk $200 to win $100.
Why it matters
Laying teaches learners the bookmaker's side of every bet, and it is essential for exchange trading, hedging, and matched-betting techniques that professionals and cautious bettors rely on.