Pokernews.com
Bad Beat
When a hand is beaten by a lucky draw.
Dan's Poker Dictionary:
Bad Beat
Different people seem to feel differently about what counts as a bad beat. One thing is certain: you have to lose the hand. What makes the beat bad? Maybe one or all of the following: you lose in a situation where you're a very big favorite; you lose with a hand you couldn't possibly have been expected to fold; you lose so improbably you feel compelled to tell the story multiple times; you lose to a player who couldn't have beat you without misplaying the hand (but who was astoundingly lucky); you lose in a way that seemed inconceivable until you saw it happen; or more than two experienced players at your table say, "ouch."
Here's an example. Say you're playing hold'em, you hold AA, the flop comes A55, and someone holding 98, suited with one of the fives, catches two perfect cards for a straight flush, you have suffered a very painful bad beat. The guy holding 55 is in a similar position, only more so.
The phrase "bad beat" is heard often in the phrase "bad beat stories," because many poker players, especially (but not exclusively) occasional or inexperienced players, love to tell stories about how rotten their luck was. Some people don't mind listening, or even enjoy these stories. Other people (especially jaded poker veterans who are pretty sure they've heard and seen it all) would sooner sit through eight hours of root canal surgery than listen to one bad beat story. Don't take it personally.
Another phrase you'll hear is "bad beat jackpot." Some games have jackpots for particular types of bad beats.
After that bad beat I put on him, he went on tilt for about six months.
Poker Encyclopedia
bad beat
An event in which a player with a high expectation of winning the pot loses. This expectation may be based on having an unusually strong hand beaten by an even stronger one, or by having an opposing player make an extremely unlikely draw. "Bad beat stories" are frequent topics of conversation at poker tables. Lou Krieger started a tradition among some players of charging $1 to listen to one. In some casinos there is a "bad beat jackpot" awarded to a player who suffers a particular beat, for example, having four of a kind beaten.
Doyle Brunson:
"When you get a big hand cracked (beaten) by someone who was a big dog against you and made his longshot draw... you're said to have had a bad beat."
Would you call the following a bad beat?
You have AhAd. Opponent has AK spades. Flop is Ac-10s-Js. Turn is 10h. River is Qs. Is that a bad beat?