The Collaborative International Dictionary of English v.0.48:

Costume \Cos"tume`\ (k?s"t?m` or k?s-t?m"), n. [F. costume, It.
   costume custom, dress, fr. L. consuetumen (not found), for
   consuetudo custom. See Custom, and cf. Consuetude.]
   1. Dress in general; esp., the distinctive style of dress of
      a people, class, or period.
      [1913 Webster]

   2. Such an arrangement of accessories, as in a picture,
      statue, poem, or play, as is appropriate to the time,
      place, or other circumstances represented or described.
      [1913 Webster]

            I began last night to read Walter Scott's Lay of the
            Last Minstrel . . . .I was extremely delighted with
            the poetical beauty of some parts . . . .The
            costume, too, is admirable.           --Sir J.
                                                  Mackintosh.
      [1913 Webster]

   3. A character dress, used at fancy balls or for dramatic
      purposes.
      [1913 Webster]

WordNet (r) 3.0 (2006):

costume
    n 1: the attire worn in a play or at a fancy dress ball; "he won
         the prize for best costume"
    2: unusual or period attire not characteristic of or appropriate
       to the time and place; "in spite of the heat he insisted on
       his woolen costume"
    3: the prevalent fashion of dress (including accessories and
       hair style as well as garments)
    4: the attire characteristic of a country or a time or a social
       class; "he wore his national costume"
    v 1: dress in a costume; "We dressed up for Halloween as
         pumpkins" [syn: costume, dress up]
    2: furnish with costumes; as for a film or play

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "costume":
   accouter, apparel, array, attire, ballet skirt, bedizenment,
   bib and tucker, blackface, buskin, cap and bells, caparison,
   clothes, clothing, clown white, disguise, drapery, dress, dressing,
   duds, equip, fashion, fatigues, feathers, fig, fit, fit out, frock,
   garb, garments, gear, getup, greasepaint, guise, habiliment, habit,
   investiture, investment, kit, linen, livery, makeup, masquerade,
   motley, outfit, practical piece, prop, property, rags, raiment,
   rig, rig out, rig up, robes, sock, sportswear, style, suit,
   theatrical makeup, threads, tights, togs, toilette, trim, turn out,
   tutu, uniform, vestment, vesture, wear, wearing apparel