A "bad beat" means that improved odds in your favour do not necessarily guarantee success. Here's an example of a bad beat: Bliinds are 600/300 & 30 Ante. The average stack is 25k. You're in the Big blind with A6 spades, UTG raises 3xBB, the guy in the Cutoff calls & you call. The flop comes: 8s,4s, Ad. You bet 2100, UTG snap calls, the Cutoff shoves his stack of 17 K allin. YOu call, UTG folds. You show your As 6s. Guy in cutoff has Jh, Qc. On the flop you're about 97.5% ahead. The odds are in your favour. The turn comes 9h, river 10c. You go in ahead & come out behind- the simplest way to describe a "badbeat".