A trapdoor is a door set into a floor or ceiling (depending on what side of the door one is on).
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.[1]
An exposed trapdoor is often called a hatch, although hatches may be horizontal. Many buildings with flat roofs have hatches that provide access to the roof; on ships, hatches—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 victim 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 who operate the Comet railcars made by Bombardier have trap doors to accommodate loading and unloading of passengers on both high-level and ground-level platforms. Amtrak's Viewliner, Amfleet, and Horizon railcar fleets all have trapdoors.
Trapdoor spiders dig out an underground nest they line with their silk, and then top it with a hinged lid, the trapdoor.[2]