Cooler
A hand you’re doomed to lose money with: strong, but running into stronger. Not a mistake, just variance.
You pick up A-A under the gun and raise. The button wakes up with K-K and three-bets. You four-bet, he shoves, you snap it off. Flop, turn, river brick, no king anywhere, and his stack slides across to you. Nobody misplayed a thing. Neither hand folds preflop, and no line dodges the result.
That's the shape: a premium hand runs into a bigger premium hand and the money goes in regardless of who's holding what. Spotting one keeps you from tearing up your strategy over a single hand. Big pairs stack off against each other often enough that it's priced in — not a sign that aces need slowplaying or that three-betting kings was a blunder.
The error runs both ways. Some players call every loss a cooler to skip the harder job of finding a real mistake, like calling off with second pair on a scary board. Others take one to heart and start flinching afterward — checking back aces, folding kings preflop. A real cooler is the rare hand you can't get away from. No amount of review turns it into a leak.