- A hinged or sliding door set into a floor or ceiling.
- (theater) Such a trap set into the floor of a stage to allow fast exits and entrances.
- (computing) A secret method of obtaining access to a program or online system; a backdoor.
- (mathematics, cryptography) The special information that permits the inverse of a trapdoor function to be easily computed.
- (mining) A door in a level for regulating the ventilating current; weather door.