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.
      [1913 Webster]

   2. (Mining) A door in a level for regulating the ventilating
      current; -- called also weather door. --Raymond.
      [1913 Webster]

   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
      Californica).
      [1913 Webster]

WordNet (r) 3.0 (2006):

trap-door spider
    n 1: American spider that constructs a silk-lined nest with a
         hinged lid