(1):

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

(2):

(n.) That which resembles a theater in form, use, or the like; a place rising by steps or gradations, like the seats of a theater.

(3):

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

(4):

(n.) 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.

(5):

(n.) A sphere or scheme of operation.