The Collaborative International Dictionary of English v.0.48:

Galleon \Gal"le*on\, n. [Sp. galeon, cf. F. galion; fr. LL.
   galeo, galio. See Galley.] (Naut.)
   A sailing vessel of the 15th and following centuries, often
   having three or four decks, and used for war or commerce. The
   term is often rather indiscriminately applied to any large
   sailing vessel.
   [1913 Webster]

         The galleons . . . were huge, round-stemmed, clumsy
         vessels, with bulwarks three or four feet thick, and
         built up at stem and stern, like castles. --Motley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

galleon
    n 1: a large square-rigged sailing ship with three or more
         masts; used by the Spanish for commerce and war from the
         15th to 18th centuries

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "galleon":
   bark, bawley, bilander, brig, brigantine, bully, buss, caravel,
   cat, catamaran, clipper, corsair, corvette, cutter, dandy, dhow,
   dromond, fishing smack, flattie, four-masted bark, four-master,
   frigate, galiot, galleass, hooker, hoy, ice yacht, junk, keelboat,
   ketch, knockabout, lateen, lateener, lorcha, lugger, nobby,
   ocean racer, outrigger, pilot boat, pinnace, piragua, pirogue,
   pram, pungy, racing yacht, rigger, sailing auxiliary,
   sailing canoe, sailing dinghy, sailing packet, sailing trawler,
   sampan, sandbagger, schooner, scooter, shallop, sharpie,
   shipentine, skipjack, sloop, smack, smack boat, snow, tartan,
   topsail schooner, trimaran, well smack, whaler, wool-clipper,
   xebec, yacht, yawl