Definition of a bad beat
A bad beat is when a player loses a hand when he was a favorite. The expression could be translated as "taking a hard hit" or "taking a bad hit". For example, you have a pair of aces on the turn. Your opponent has a pair of kings. The board is A-5-7-K. You both go all in. At this point, you know that you have a little over 97% chance of winning the pot. The river falls and it's another king. Your opponent wins the pot with a square. You will then say that you got a bad beat.
What is an bad beat Jackpot?
Some online poker's offer their players an bad beat jackpot option. At these particular tables (identified as such), in addition to the usual commission, an extra commission will be taken on every pot that is sent directly to a progressive grand prize. If a player loses a hand with a poker-of-a-Kind 8 or better (the nature of the hand may vary from room to room) with two cards in hand (two 8s), the player who loses the hand will receive 50% of the prize pool, the player who wins the hand will receive 25% of the prize pool, while the remaining 25% will be split equally among all other players at the table.