The zwischenzug (German for "in-between move") is an unexpected move inserted into an apparently forced sequence — such as a series of exchanges — before responding to the opponent’s last move.
Instead of playing the "natural" reply (recapturing a piece, getting out of check, etc.), the player first introduces a more pressing threat — typically a check, a winning capture, or a decisive attack — forcing the opponent to deal with it. Only then does the original sequence resume, but in a completely different position. For example: White captures a piece, Black seems obliged to recapture, but instead plays an intermediate check that wins a tempo or an extra piece.
In practice, before automatically recapturing or continuing an exchange, always ask yourself: "Is there an even stronger move available right now?" Developing this habit of looking for a zwischenzug can turn a neutral sequence into a material gain or a decisive advantage.