Definitions: Discovered Attack

This might be called 'removing a friendly pin'.

Discovered Attack: A piece is moved to unblock a line of attack against a target.

Euwe vs Thomas, Hastings, 1934 (ECM Position #51)
White to move

White plays 1. Bd5 discovering an attack against the Rook at f8 and simultaneously attacking the Queen. If 1. ... Rxf2, 2. Qg8#, and if the Bishop is captured, 2. Rxf8+ followed by 3. Qxg8#.