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