The Collaborative International Dictionary of English v.0.48:

overcoat \o"ver*coat`\, n. [Cf. Surcoat.]
   A coat worn over the other clothing; a greatcoat; a topcoat.
   [1913 Webster]

WordNet (r) 3.0 (2006):

overcoat
    n 1: a heavy coat worn over clothes in winter [syn: greatcoat,
         overcoat, topcoat]
    2: an additional protective coating (as of paint or varnish)
       [syn: overcoat, overcoating]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "overcoat":
   Inverness, benjamin, capote, chesterfield, cloth coat, coat,
   dreadnought, duster, fur-lined coat, fur-trimmed coat, greatcoat,
   jacket, long coat, mac, mackintosh, paletot, raglan, raincoat,
   slicker, surtout, tarpaulin, topcoat, trench coat, ulster,
   waterproof, wrap-around