The Collaborative International Dictionary of English v.0.48:

Planchette \Plan`chette"\, n. [F. See Planchet.]
   1. A circumferentor. See Circumferentor.
      [1913 Webster]

   2. A small tablet of wood supported on casters and having a
      pencil attached. The characters produced by the pencil on
      paper, while the hand rests on the instrument and it is
      allowed to move, are sometimes translated as of oracular
      or supernatural import.
      [1913 Webster]

WordNet (r) 3.0 (2006):

planchette
    n 1: a triangular board supported on casters; when lightly
         touched with the fingertips it is supposed to spell out
         supernatural (or unconscious) messages