The Collaborative International Dictionary of English v.0.48:

Trap \Trap\, n. [Sw. trapp; akin to trappa stairs, Dan. trappe,
   G. treppe, D. trap; -- so called because the rocks of this
   class often occur in large, tabular masses, rising above one
   another, like steps. See Tramp.] (Geol.)
   An old term rather loosely used to designate various
   dark-colored, heavy igneous rocks, including especially the
   feldspathic-augitic rocks, basalt, dolerite, amygdaloid,
   etc., but including also some kinds of diorite. Called also
   trap rock.
   [1913 Webster]

   Trap tufa, Trap tuff, a kind of fragmental rock made up
      of fragments and earthy materials from trap rocks.
      [1913 Webster]