Irreversible move

An irreversible move changes something that cannot be restored merely by moving the pieces back. The term does not mean the game is strategically won or that a similar arrangement can never reappear. It means the legal or technical state cannot become exactly the same again.

Narrow meaning for the fifty-move rule

For the rule allowing a draw claim after fifty moves by each player without a pawn move or capture, the moves that restart the count are captures and pawn moves. A double pawn push belongs to this group.

Broader meaning for repetition and software

Moves that permanently remove castling rights, the historical permission to move the king and a rook through that special move, are also called irreversible, even when they are neither captures nor pawn moves. Technical systems may additionally classify a move that gives up an en passant right as irreversible. Even if the pieces later return to the same squares, the available legal moves no longer match the earlier position.

The intended meaning therefore matters. A non-capturing king move may be irreversible for repetition because it removes castling rights, yet it does not restart the fifty-move count. A capture does both.