The Collaborative International Dictionary of English v.0.48:

Planchet \Planch"et\, n. [F. planchette a small board, dim. of
   planche. See Planch.]
   A flat piece of metal; especially, a disk of metal ready to
   be stamped as a coin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

planchet
    n 1: a flat metal disk ready for stamping as a coin [syn:
         planchet, coin blank]