Back Bet
The traditional bet that an outcome WILL happen, paired on an exchange with someone laying it.
Definition
Backing is the familiar form of betting: you stake on an outcome to occur and win if it does. On a betting exchange every back bet must be matched by another user laying the same outcome, whereas at a traditional bookmaker the book itself takes the other side. Your risk is limited to your stake, and your potential profit is stake x (odds - 1). Back and lay are the two complementary sides of any wager.
Worked example
You back Team A at 3.0 for $100. If Team A wins you profit $200 (stake x (3.0 - 1)); if it loses you forfeit the $100 stake.
Why it matters
Back is the baseline every bettor starts from, and pairing it explicitly with lay helps learners grasp that each bet has two sides and a counterparty who profits when they lose.