A bad beat has nothing to do with a jackpot. . . for that, see BadBeatJackpot.
A bad beat is when a hand that has no business playing beats a better hand by catching lucky cards. Not every lucky river makes for a bad beat -- it has to be a river seen against any reasonable expectation.
Example
If someone comes in with
and their opponent has
and the board comes down
and the AKs bets the flop and the turn, then then 43s hand has no business playing. Calling such bets is downright awful.
What can happen?
But they might play. If the full board ends up like this:
Then the 43s hand would win over the aces and kings. You would have a classic "Bad Beat."
Here's another bad beat
Again, the 43s hand wins.
