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