Thwomp is a trap-like enemy that first appeared in Super Mario Bros 3. They are most commonly seen on the ceilings of fortresses and castles held by Bowser's underlings. Thwomps attack by falling directly onto the player. Some Thwomps fall and rise automatically at a set rate, but most are triggered by a proximity sensor.
Related to Thwomps are the Thwimps, essentially a tiny version of the Thwomp. Thwimps first appeared in Super Mario World. Instead of being tied to the ceiling, they attack by jumping in Mario's direction in a high arc.
Thwomps have also made appearances outside the Mario platforming games. In the Mario Kart series they are able to crush racers in the Bowser's Castle levels and at Thwomp Desert in Mario Kart Wii.