trapdoor

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A hinged or sliding door in a floor, roof, or ceiling.

from The Century Dictionary.

• noun In mining, a door designed to direct air-currents in ventilating systems, or to prevent, surface wind from deranging the normal direction of air-currents.
• noun A door in a floor or roof which when shut is flush, or nearly so, with what surrounds it.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Arch.) A lifting or sliding door covering an opening in a roof or floor.
• noun (Mining) A door in a level for regulating the ventilating current; -- called also weather door.
• noun (Zoöl.) any one of several species of large spiders which make a nest consisting of a vertical hole in the earth, lined with a hinged lid, like a trapdoor. Most of the species belong to the genus Cteniza, as the California species (Cteniza Californica).

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A hinged or sliding door set into a floor or ceiling.
• noun theater Such a trap set into the floor of a stage to allow fast exits and entrances.
• noun computing A secret method of obtaining access to a program or online system; a backdoor.
• noun mathematics, cryptography The special information that permits the inverse of a trapdoor function to be easily computed.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a hinged or sliding door in a floor or ceiling