trapdoor

[trap-dawr, -dohr] /ˈtræpˈdɔr, -ˈdoʊr/
noun
1.
a door flush with the surface of a floor, ceiling, or roof.
2.
the opening that it covers.
adjective
3.
of, pertaining to, or like a trapdoor.
Also, trap door.
Origin
1325-75; Middle English trappe dore. See trap1, door
British Dictionary definitions for trap door

trap door

noun
1.
a door or flap flush with and covering an opening, esp in a ceiling
2.
the opening so covered
trap door in Technology


Or "trapdoor" 1. back door.
2. trap-door function