A trapdoor is a sliding or hinged door, flush with the surface of a floor, roof, or ceiling, or in the stage of a theatre. A hatch, an opening which may also be in a wall and need not be flush with the surface, is similar; in some cases either name is applicable.
Originally, trapdoors were sack traps in mills, and allowed the sacks to pass up through the mill while naturally falling back to a closed position.
Many buildings with flat roofs have hatches that provide access to the roof; on ships, hatches—usually not flush, and never called trapdoors—provide access to the deck. Cargo ships, including bulk carriers, have large hatches for access to the holds. A small door in a wall, floor or ceiling used to gain access to equipment is called an access hatch. Hidden trapdoors occasionally appear in fiction, either as entrances to secret passageways, or as literal traps into which a helpless pedestrian may fall if he or she happens to stand on one.
Most 19th- and 20th Century gallows featured a trapdoor, usually with two flaps. The victim was placed at the join. The edge of a trapdoor farthest from the hinge accelerates faster than gravity, so that the prisoner does not hit the flaps but falls freely.
The term trapdoor also refers to a plate in the vestibule of a railcar that permits access to high-level platforms when lying flat against the floor of the car and can be flipped open to expose steps for accessing ground-level platforms. Many American commuter railroads which operate the Comet railcars made by Bombardier have trapdoors to accommodate passengers boarding and alighting on both high-level and ground-level platforms. Amtrak's Viewliner, Amfleet, and Horizon railcar fleets all have trapdoors.
Trapdoor spiders hide in an underground nest they line with their silk, and then conceal it with a hinged silk lid, the trapdoor.