The Collaborative International Dictionary of English v.0.48:

Gown \Gown\ (goun), n. [OE. goune, prob. from W. gwn gown, loose
   robe, akin to Ir. gunn, Gael. g[`u]n; cf. OF. gone, prob. of
   the same origin.]
   1. A loose, flowing upper garment; especially:
      (a) The ordinary outer dress of a woman, especially one
          that is full-length/ex>.
      (b) The official robe of certain professional men and
          scholars, as university students and officers,
          barristers, judges, etc.; hence, the dress of peace;
          the dress of civil officers, in distinction from
          military.
          [1913 Webster]

                He Mars deposed, and arms to gowns made yield.
                                                  --Dryden.
      (c) A loose wrapper worn by gentlemen within doors; a
          dressing gown.
          [1913 Webster]

   2. Any sort of dress or garb.
      [1913 Webster]

            He comes . . . in the gown of humility. --Shak.
      [1913 Webster]

   3. An evening gown.
      [PJC]

   4. The students and faculty of a college and university, as
      opposed to the local inhabitants not connected to the
      university; -- used often in the phrase "town and gown",
      referring to interactions between the university and the
      local townspeople; as, a town and gown dispute.
      [PJC]

WordNet (r) 3.0 (2006):

gown
    n 1: a woman's dress, usually with a close-fitting bodice and a
         long flared skirt, often worn on formal occasions
    2: the members of a university as distinguished from the other
       residents of the town in which the university is located;
       "the relations between town and gown are always sensitive"
    3: lingerie consisting of a loose dress designed to be worn in
       bed by women [syn: nightgown, gown, nightie, night-
       robe, nightdress]
    4: protective garment worn by surgeons during operations [syn:
       gown, surgical gown, scrubs]
    5: outerwear consisting of a long flowing garment used for
       official or ceremonial occasions [syn: gown, robe]
    v 1: dress in a gown

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "gown":
   bonnet, boot, breech, cap, cloak, coat, coif, dress, duds, frock,
   garment, hat, hood, jacket, jupe, mantle, rag, raiment, robe,
   shirt, shoe, skirt, sock, stocking, togs, vestment, vesture