The Collaborative International Dictionary of English v.0.48:

Doubloon \Doub*loon"\, n. [F. doublon, Sp. doblon. See Double,
   a., and cf. Dupion.]
   A Spanish gold coin, no longer issued, varying in value at
   different times from over fifteen dollars to about five. See
   Doblon in Sup.
   [1913 Webster]

WordNet (r) 3.0 (2006):

doubloon
    n 1: a former Spanish gold coin

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "doubloon":
   coin, crown, double eagle, ducat, 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