The Collaborative International Dictionary of English v.0.48:
Trapdoor \Trap"door`\, n.
1. (Arch.) A lifting or sliding door covering an opening in a
roof or floor.
2. (Mining) A door in a level for regulating the ventilating
current; -- called also weather door. --Raymond.
Trapdoor spider (Zool.), 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