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.
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   2. Any room adapted to the exhibition of any performances
      before an assembly, as public lectures, scholastic
      exercises, anatomical demonstrations, surgical operations,
      etc.
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   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.
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            Shade above shade, a woody theater
            Of stateliest view.                   --Milton.
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   4. A sphere or scheme of operation. [Obs.]
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            For if a man can be partaker of God's theater, he
            shall likewise be partaker of God's rest. --Bacon.
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   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]