An arb is the colloquial term for an arbitrage bet — a situation where odds discrepancies between bookmakers allow a bettor to cover all outcomes of an event and guarantee a profit. The word is used both as a noun ("I found an arb") and a verb ("I arbed that match").
Arbs appear most frequently in markets with only two outcomes (tennis, head-to-head matchups) because covering two sides requires finding a discrepancy between just two prices. Three-outcome markets (football 1X2) require wider discrepancies to overcome the bookmaker margins on all three sides.
The practice of arbing is entirely legal but actively discouraged by bookmakers. Accounts that show consistent arbing patterns are restricted through stake limits, known as gubbing. Experienced arbers spread activity across many bookmakers, mix arbing with recreational bets, and exploit soft bookmaker accounts before they are restricted.
Speed is the defining skill in arbing. Odds compilers correct pricing errors within minutes, so arbs that show a 3% profit margin can vanish in seconds. The most profitable arbers automate the discovery and stake calculation process.
Example
You find that Bookmaker A prices Andy Murray to beat Novak Djokovic at 2.15, and Bookmaker B prices Djokovic at 2.05. Combined implied probability: (1/2.15) + (1/2.05) = 46.5% + 48.8% = 95.3%. You stake £465 on Murray and £488 on Djokovic (total £953). Whoever wins, your return is approximately £1,000 — a profit of ~£47 (about 4.9%) regardless of outcome.