Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

   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.

            Shade above shade, a woody theater Of stateliest
            view.                                 --Milton.

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

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

   5. A place or region where great events are enacted; as, the
      theater of war.

Source : WordNet®

theatre
     n 1: a building where theatrical performances or motion-picture
          shows can be presented; "the house was full" [syn: {theater},
           {house}]
     2: the art of writing and producing plays [syn: {dramaturgy}, {dramatic
        art}, {dramatics}, {theater}]
     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 of operations}]