Playing worse than you know how to play because of how you feel. The word covers a range of states — anger after a bad beat is only the most obvious one — and the common thread is that the decision-making changes while the player's confidence in it does not.
The forms are more varied than the stereotype. There is the obvious kind, where a player who has just lost a large pot starts opening every hand to get it back. There is the quieter kind: playing on well past the point of fatigue, calling river bets out of curiosity, refusing to fold to one particular opponent, or turning cautious and passive after a loss and folding hands that are clear calls. Winner's tilt is real too — a good run producing a loosening of standards that would never survive a sober look.
The costly part is not the emotion; it is that the internal monitor is exactly what the emotion takes offline. Tilt does not feel like tilt at the time. It feels like a justified adjustment, or like being owed something.
Because self-assessment fails in the moment, the countermeasures have to be set in advance and be mechanical: a stop-loss decided before sitting down, a fixed break after any pot above a certain size, an agreement with yourself to end the session at a set time regardless of the result. The value of a rule written beforehand is precisely that following it requires no judgement from the person who has temporarily lost access to theirs.