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An accumulator is a single bet combining four or more selections, where the returns from each winning leg roll onto the next. Every selection must win for it to pay out, so the odds multiply — a four-fold of 2/1 shots returns 80/1 — but so does the risk: one loser anywhere and the whole bet is gone. Accas offer big payouts for small stakes, which is exactly why the combined chance of all legs landing is usually far lower than it feels.
Technically four or more. Two selections is a double, three is a treble, and four or more is an accumulator (four-fold, five-fold, and so on). They all work the same way: every leg must win and the returns compound from one leg to the next.
Multiply the decimal odds of every leg together, then multiply by your stake. Three legs at 2.0, 3.0 and 1.5 give 2.0 × 3.0 × 1.5 = 9.0, so a £10 stake returns £90. The accumulator calculator does it instantly for any number of legs.
Because the chance of every leg winning multiplies down fast. Four selections each with a realistic 30% chance have only about a 0.8% combined chance of all landing (0.3 × 0.3 × 0.3 × 0.3). The big advertised payout reflects how unlikely the bet is, not how good value it is.
As a small-stake, high-variance flutter, they're fine and fun. As a profit strategy they rarely are, because the bookmaker's margin is compounded on every leg. Singles on genuine value selections are almost always the more rational long-term play.
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