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

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "ducat":
   coin, crown, double eagle, doubloon, eagle, five-dollar gold piece,
   gold piece, guinea, half crown, half eagle, hard money, moidore,
   napoleon, piece, piece of money, piece of silver, pound sovereign,
   roll of coins, rouleau, sovereign, specie, ten-dollar gold piece,
   twenty-dollar gold piece



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DUCAT. The name of a foreign coin. The ducat of Naples shall be estimated in
the computations of customs, at eighteen cents. Act of May 22, 1846.