A chess term, from the German for "in-between move". Zwischenzug refers to a move which is unexpectedly inserted in the middle of an apparently forced sequence of moves. Perhaps the most common circumstance where a zwischenzug arises is during a trade of pieces; a player may capture a piece, expecting an immediate recapture, when in fact his opponent can seize the initiative by playing an attacking move before recapturing. Here is an example position where Black falls prey to a zwischenzug:

+---+---+---+---+---+---+---+---+
|   |   |BK |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
|BP |BP |BP |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
|   |   |   |BQ |BB |   |BR |   |
+---+---+---+---+---+---+---+---+
|   |   |   |   |WQ |   |   |BN |
+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
|   |   |   |WN |   |WR |   |   |
+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   |WP |WP |
+---+---+---+---+---+---+---+---+
|   |   |   |   |   |WK |   |   |
+---+---+---+---+---+---+---+---+

Here Black, since he is ahead on material, decides to trade off queens and rooks to enter a favourable endgame. He plays 1. ...Qxe5?? expecting the knight to recapture, when he could play Rf6 forcing the exchange of rooks. White, however, finds 2. Rf8+!, forcing the king to d7, after which 3. Nxe5+ wins Black's rook. (It is quite common for the zwischenzug to be a move giving check; the German term for this is zwischenschach, meaning "in-between check".)

The moral: assume nothing at the chessboard; quickly calculate the variations even in an "obvious" sequence of moves, lest your opponent surprises you with a tactic like that above.