A hinged or sliding door set into a floor or ceiling.
The special information that permits the inverse of a trapdoor function to be easily computed.
Such a trap set into the floor of a stage to allow fast exits and entrances.
A door in a level for regulating the ventilating current; a weather door.
A secret method of obtaining access to a program or online system; a backdoor.
A small gate or door, especially one built into a large one.