The Collaborative International Dictionary of English v.0.48:

Curtain \Cur"tain\ (k[^u]r"t[i^]n; 48), n. [OE.cortin,
   curtin,fr. OF. cortine, curtine, F. courtine, LL. cortina,
   curtian (in senses 1 and 2), also, small court, small
   inclosure surrounded by walls, from cortis court. See
   Court.]
   [1913 Webster]
   1. A hanging screen intended to darken or conceal, and
      admitting of being drawn back or up, and reclosed at
      pleasure; esp., drapery of cloth or lace hanging round a
      bed or at a window; in theaters, and like places, a
      movable screen for concealing the stage.
      [1913 Webster]

   2. (Fort.) That part of the rampart and parapet which is
      between two bastions or two gates. See Illustrations of
      Ravelin and Bastion.
      [1913 Webster]

   3. (Arch.) That part of a wall of a building which is between
      two pavilions, towers, etc.
      [1913 Webster]

   4. A flag; an ensign; -- in contempt. [Obs.] --Shak.
      [1913 Webster]

   Behind the curtain, in concealment; in secret.

   Curtain lecture, a querulous lecture given by a wife to her
      husband within the bed curtains, or in bed. --Jerrold.
      [1913 Webster]

            A curtain lecture is worth all the sermons in the
            world for teaching the virtues of patience and
            long-suffering.                       --W. Irving.

   The curtain falls, the performance closes.

   The curtain rises, the performance begins.

   To draw the curtain, to close it over an object, or to
      remove it; hence:
      (a) To hide or to disclose an object.
      (b) To commence or close a performance.

   To drop the curtain, to end the tale, or close the
      performance.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Curtain \Cur"tain\, v. t. [imp. & p. p. Curtained
   (k[^u]r"t[i^]nd; 48); p. pr. & vb. n. Curtaining.]
   To inclose as with curtains; to furnish with curtains.
   [1913 Webster]

         So when the sun in bed
         Curtained with cloudy red.               --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

curtain
    n 1: hanging cloth used as a blind (especially for a window)
         [syn: curtain, drape, drapery, mantle, pall]
    2: any barrier to communication or vision; "a curtain of
       secrecy"; "a curtain of trees"
    v 1: provide with drapery; "curtain the bedrooms"