Bad beat
A bad beat is when a hand that was a strong favorite to win loses to an opponent who catches a lucky card that completes their unlikely hand.
The phrase describes a painful loss: you had the better hand and the math on your side, then your opponent caught exactly the card they needed on a later street to overtake you. It is less about playing badly and more about getting unlucky after the chips were already in, which is why players groan "bad beat" instead of "my mistake."
It usually comes up on the turn or the river, when a draw that was unlikely to complete actually gets there. Holding the best hand only means you were ahead at that moment, not that you were guaranteed the pot, so even a big favorite loses sometimes. The wider the gap between how far ahead you were and the result, the more it feels like a bad beat.
For beginners, the key takeaway is that one bad beat does not mean you played the hand wrong. If you got your chips in as the favorite, you made a good decision and the cards simply did not cooperate this time, which is the swingy nature of the game (sometimes called variance). The trap to avoid is letting a bad beat rattle you into reckless play afterward, a frustrated state players call being on tilt; take a breath and keep making the same solid decisions.
Example
You hold A-A and your opponent holds 7-7. The flop comes K-Q-2, missing them completely, so your aces are a huge favorite. Then a 7 falls on the river, giving them a set of sevens (a pocket pair plus one matching board card) to beat your aces. They had only two cards in the deck that could save them and caught one of them, a classic bad beat for the favorite.