The Collaborative International Dictionary of English v.0.48:

Ducat \Duc"at\, n. [F. ducat, It. ducato, LL. ducatus, fr. dux
   leader or commander. See Duke.]
   A coin, either of gold or silver, of several countries in
   Europe; originally, one struck in the dominions of a duke.
   [1913 Webster]

   Note: The gold ducat is generally of the value of nine
         shillings and four pence sterling, or somewhat more
         that two dollars. The silver ducat is of about half
         this value.
         [1913 Webster]

WordNet (r) 3.0 (2006):

ducat
    n 1: formerly a gold coin of various European countries