WordNet (r) 3.0 (2006):

trap door
    n 1: a hinged or sliding door in a floor or ceiling

The Free On-line Dictionary of Computing (19 January 2023):

trap door

   Or "trapdoor" 1. back door.

   2. trap-door function


The Jargon File (version 4.4.7, 29 Dec 2003):

trap door
 n.

    (alt.: trapdoor)

    1. Syn. back door — a Bad Thing.

    2. [techspeak] A trap-door function is one which is easy to compute but
    very difficult to compute the inverse of. Such functions are Good Things
    with important applications in cryptography, specifically in the
    construction of public-key cryptosystems.