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; a weather door.

(cryptography, transitive) To subvert (a cryptographic scheme) by means of a secret parameter that makes the trapdoor function easy to compute.

(surgery, intransitive) To form a trapdoor deformity.