In chess, a "blunder" is an extremely bad move, typically one that drastically shifts a player's situation from "winning" to "drawn" or even "losing" in a single move. In chess annotation, a move considered to be a blunder is typically marked with a double question mark (??) after the move notation.
A blunder is contrasted with other types of bad moves of lesser impact - the "mistake" and the "inaccuracy."
For much of chess history, which moves counted as blunders was often quite subjective. However, in recent times, with the advent of computer evaluation by sophisticated chess engines, what counts as a blunder is often precisely defined. Typically a blunder is defined by computers as a move that loses the player three pawns or more of relative advantage compared to what the computer believes to be the best possible move.