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"

Moby Thesaurus II by Grady Ward, 1.0:

287 Moby Thesaurus words for "curtain":
   Z, abatis, act, act drop, advanced work, afterpiece, apodosis,
   apply to, asbestos, asbestos board, awning, backdrop, balistraria,
   bamboo curtain, bank, banquette, barbed-wire entanglement,
   barbican, barricade, barrier, barrier of secrecy, bartizan,
   bastion, batten, battlement, beach umbrella, becloud, befog, bit,
   blackout, blanket, blind, block, blockade, border, bottleneck,
   breastwork, bulwark, camouflage, canopy, casemate, catastrophe,
   catch, ceasing, censorship, cessation, chaser, cheval-de-frise,
   circumvallation, cloak, cloth, clothe, cloud, coat, coda, conceal,
   conclusion, consummation, contravallation, cope, cordon, coulisse,
   counterscarp, counterweight, cover, cover up, coverage, covering,
   covert, coverture, cowl, cowling, crack of doom, culmination,
   curtain board, curtain call, curtain raiser, curtains, cyclorama,
   death, decease, decor, demibastion, denouement, destination,
   destiny, determent, deterrent, difficulty, dike, disguise,
   dissemble, distract attention from, divertimento, divertissement,
   doom, drape, drapery, draw the curtains, drawback, drawbridge,
   drop, drop curtain, earthwork, eclipse, effect, enclosure, end,
   end point, ending, ensconce, enshroud, entanglement, envelop,
   envoi, epilogue, escarp, escarpment, eschatology, exode, exodus,
   expiration, expository scene, fate, fence, fieldwork, film,
   final solution, final twitch, final words, finale, finality, finis,
   finish, fire curtain, flat, flipper, fortalice, fortification,
   glacis, gloss over, goal, guise, hang-up, hanging, hazard, hide,
   hitch, hoke act, hood, housing, hurdle, hush-up, interlude,
   intermezzo, intermission, introduction, iron curtain,
   ironbound security, izzard, joker, keep under cover, last,
   last breath, last gasp, last things, last trumpet, last words,
   latter end, lay on, lay over, light shield, loophole, lunette,
   machicolation, mantelet, mantle, mask, merlon, mound, muffle,
   number, oath of secrecy, obduce, obfuscate, objection, obscure,
   obstacle, obstruction, obstructive, occult, official secrecy,
   omega, one small difficulty, outwork, overlay, overshadow,
   overspread, palisade, pall, parados, parapet, parasol, payoff,
   period, peroration, portcullis, postern gate, prologue, put on,
   quietus, rag, rampart, ravelin, redan, redoubt, repression,
   resolution, resting place, routine, rub, sally port, scarp, scene,
   scenery, sconce, screen, scum, seal of secrecy, security, shade,
   shader, shadow, shelter, shield, shroud, shtick, shutter,
   side scene, sketch, skit, slur over, smothering, snag,
   song and dance, spread over, stage screw, stand-up comedy act,
   stifling, stockade, stoppage, stopping place, striptease,
   stumbling block, stumbling stone, sunblind, sunshade, superimpose,
   superpose, suppression, swan song, tab, tableau, teaser, tenaille,
   term, terminal, termination, terminus, tormentor, transformation,
   transformation scene, turn, umbrella, vallation, vallum, varnish,
   veil, veil of secrecy, vestment, whitewash, windup, wing, wingcut,
   woodcut, work, wraps



Easton's 1897 Bible Dictionary:

Curtain
   (1.) Ten curtains, each twenty-eight cubits long and four wide,
   made of fine linen, also eleven made of goat's hair, covered the
   tabernacle (Ex. 26:1-13; 36:8-17).

     (2.) The sacred curtain, separating the holy of holies from
   the sanctuary, is designated by a different Hebrew word
   (peroketh). It is described as a "veil of blue, and purple, and
   scarlet, and fine twined linen of cunning work" (Ex. 26:31; Lev.
   16:2; Num. 18:7).

     (3.) "Stretcheth out the heavens as a curtain" (Isa. 40:22),
   is an expression used with reference to the veil or awning which
   Orientals spread for a screen over their courts in summer.
   According to the prophet, the heavens are spread over our heads
   as such an awning. Similar expressions are found in Ps. 104:2l;
   comp. Isa. 44:24; Job 9:8.