Trapdoor

noun

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