In chess, a "mistake" is a really bad move that significantly weakens the player's position. In chess notation, a move considered to be a "mistake" is typically marked with a single question mark (?) after the move notation.
A "mistake" is worse than the mildly negative "inaccuracy," but is nowhere near as bad as the utterly terrible "blunder."
For much of chess history, deciding which moves counted as mistakes was often quite subjective. However, in recent times, with the advent of computer evaluation by sophisticated chess engines, what counts as a mistake is often precisely defined. Typically a mistake is defined by computers as a move that loses the player between 1 and 3 pawns of relative advantage compared to what the computer believes to be the best possible move.