Lay Bet
In short: A bet against an outcome occurring, placed on a betting exchange. Functionally equivalent to taking the role of the bookmaker — accepting another bettor's stake on an outcome and paying out if the outcome occurs, keeping the stake if it doesn't.
Also known as: laying, backing the field
On a traditional sportsbook, every bet is a back bet — you wager on an outcome to occur. On a betting exchange, you can also lay an outcome, which is functionally the inverse: you’re offering the bet that another user takes. If they bet $100 on Team A to win at +200, you lay Team A by posting that price and accepting their $100 — owe them $200 if Team A wins, keep $100 if Team A loses. Laying Team A at +200 is mathematically equivalent to backing Team B at -200, but the bet sits in the order book differently and can produce slightly better effective pricing depending on liquidity.