In chess, an "inaccuracy" is bad move that slightly weakens the player's position. In chess notation, a move considered to be an inaccuracy is typically marked with a question mark followed by an exclamation point (?!) after the move notation.
An inaccuracy is not as bad as the other two types of bad moves in chess - the really bad "mistake" and the utterly terrible "blunder."
For much of chess history, deciding which moves counted as inaccuracies was often quite subjective. However, in recent times, with the advent of computer evaluation by sophisticated chess engines, what counts as an inaccuracy is often precisely defined. Typically an inaccuracy is defined by computers as a move that loses the player between 0.5 and 1 pawns of relative advantage compared to what the computer believes to be the best possible move.