Definitions: Clearance

Usually referring to single squares, but lines are also cleared so pieces can penetrate the enemy position.

Clearance: Moving a piece so the square on which it stood or the line it blocked can be used by other pieces.

Reinfeld position #548
White to move

White plays 1. Qxe5 clearing the f6 square for the 'Family Fork' 2. Nf6+.

Here's a very famous and entertaining example from Pillsbury.

Pillsbury vs Em. Lasker, Nuremberg, 1896
White to move

21. f5 clearing the f4 square for the Knight 21. ... gxf5 22. Nf4 h4 preventing the Queen's penetration by Qg3-g7 23. Ra1 Be7 24. Rxa4 decoying the Bishop at d7 from the protection of the King side 24. ... Bxa4 25. Ndxe6 fxe6 26. Nxe6 now if say 26. ... Qc8 27. Qxf5 is crushing. Lasker actually played 26. ... Bd7 to prolong the game but couldn't save it (based on Reti's notes from 'Masters of the Chessboard').