Art where the meaning is contained not in the aesthetics of the piece but in its effect on the viewer through their interaction with the piece.

Classic example: an installation where the patrons file through a dim corridor toward a brilliant gallery, only to stumble on a cement block embedded in the floor at the doorway, and, presumably cursing and rubbing their toes, look around to find the gallery empty.

The intended effect is an oscillation in the patrons' minds between annoyance at the trap, bewilderment at the lack of obvious Art in the room, and delight at the realization that the art was not in the room but in them, and between themselves and the artist, who at that very moment might be asleep somewhere, under a parasol.