What is an arbitrage / arb in sports betting?

What is an arbitrage / arb in sports betting? Exploiting odds differences to guarantee profit regardless of outcome. In plain terms: two books disagree hard enough that backing both sides costs less than either payout. Every outcome returns more than the total staked — on paper. “Last night was a great day — wound up having a day to myself without a sweat. Picked off some arb plays that guaranteed landing in the green.” Let's use this example. Book A has the over at plus one ten; Book B has the under at plus one five. Stake both sides correctly and either result returns more than the combined outlay. Real arbs are small, vanish fast, and books limit accounts that chase them — the catch is the career, not the math. Lines and prices can differ by sportsbook. Nothing here guarantees a result or tells you to bet. Lines move and books differ. Nothing here is a guarantee or a command to bet — it's how the number works.