The Collaborative International Dictionary of English v.0.48:

Theater \The"a*ter\, Theatre \The"a*tre\, n. [F. th['e][^a]tre,
   L. theatrum, Gr. ?, fr. ? to see, view; cf. Skr. dhy[=a] to
   meditate, think. Cf. Theory.]
   1. An edifice in which dramatic performances or spectacles
      are exhibited for the amusement of spectators; anciently
      uncovered, except the stage, but in modern times roofed.
      [1913 Webster]

   2. Any room adapted to the exhibition of any performances
      before an assembly, as public lectures, scholastic
      exercises, anatomical demonstrations, surgical operations,
      etc.
      [1913 Webster]

   3. That which resembles a theater in form, use, or the like;
      a place rising by steps or gradations, like the seats of a
      theater. --Burns.
      [1913 Webster]

            Shade above shade, a woody theater
            Of stateliest view.                   --Milton.
      [1913 Webster]

   4. A sphere or scheme of operation. [Obs.]
      [1913 Webster]

            For if a man can be partaker of God's theater, he
            shall likewise be partaker of God's rest. --Bacon.
      [1913 Webster]

   5. A place or region where great events are enacted; as, the
      theater of war.
      [1913 Webster] Theatin

WordNet (r) 3.0 (2006):

theater
    n 1: a building where theatrical performances or motion-picture
         shows can be presented; "the house was full" [syn:
         theater, theatre, house]
    2: the art of writing and producing plays [syn: dramaturgy,
       dramatic art, dramatics, theater, theatre]
    3: a region in which active military operations are in progress;
       "the army was in the field awaiting action"; "he served in
       the Vietnam theater for three years" [syn: field, field of
       operations, theater, theater of operations, theatre,
       theatre of operations]

Moby Thesaurus II by Grady Ward, 1.0:

192 Moby Thesaurus words for "theater":
   Broadway, DMZ, Elizabethan theater, Globe Theatre, Greek theater,
   aceldama, agora, amphitheater, amusement park, arena,
   arena theater, assembly hall, athletic field, audience, auditorium,
   auditory, back, backdrop, background, ballroom, battle line,
   battle site, battlefield, battleground, bear garden, boards, bowl,
   boxing ring, bull ring, burlesque, cabaret, cafe chantant,
   cafe dansant, campus, canvas, carnival, casino, chapel, cincture,
   circle theater, circus, classroom, close, club, cockpit, coliseum,
   colosseum, combat area, combat zone, concert hall, confine,
   congregation, container, convention hall, coop, course, court,
   courtyard, curtilage, dance floor, dance hall, dancing pavilion,
   delimited field, distance, drama, enclave, enclosure, enemy line,
   entertainment industry, exhibition hall, field, field of battle,
   field of blood, firing line, floor, fold, footlights, forum,
   front line, fun-fair, gallery, ground, groundling, gym, gymnasium,
   hall, hinterland, hippodrome, house, juke joint, killing ground,
   landing beach, lecture hall, lecture room, legit, legitimate stage,
   line, line of battle, list, lists, little theater, locale,
   marketplace, mat, meetinghouse, milieu, mise-en-scene, music hall,
   night spot, nightclub, nitery, off Broadway, off-off-Broadway,
   open forum, opera, opera house, orchestra, outdoor theater,
   palaestra, pale, paling, parade ground, park, pen, pit, place,
   platform, playhouse, playland, precinct, prize ring, public square,
   purlieu, quad, quadrangle, range, rear, recitation room,
   repertory drama, resort, ring, roadhouse, scene, scene of action,
   scenery, schoolroom, seat of war, setting, shambles, show biz,
   show business, showboat, site, spectator, sphere, square,
   squared circle, stadium, stage, stage set, stage setting,
   stage world, stagedom, stageland, stock, strawhat,
   strawhat circuit, summer stock, tavern, terrain, the boards,
   the footlights, the front, the scenes, the stage, the theater,
   theater of operations, theater of war, theater world,
   theater-in-the-round, theatromania, theatron, theatrophobia,
   tilting ground, tiltyard, toft, variety, vaudeville, walk,
   wrestling ring, yard, zone of communications