1. A building, room, or outdoor structure for the presentation of plays, films, or other dramatic performances.
2. A room with tiers of seats used for lectures or demonstrations: an operating theater at a medical school.
3. 1. Dramatic literature or its performance; drama: the theater of Shakespeare and Marlowe.
   2. The milieu of actors and playwrights.
   3. The quality or effectiveness of a theatrical production: good theater; awful theater.
   4. Dramatic material or the use of such material: "His summation was a great piece of courtroom theater" (Ron Rosenbaum).
4. 1. The quality or effectiveness of a theatrical production: good theater; awful theater.
   2. Dramatic material or the use of such material: "His summation was a great piece of courtroom theater" (Ron Rosenbaum).
5. The audience assembled for a dramatic performance.
6. A place that is the setting for dramatic events.
7. A large geographic area in which military operations are coordinated: the European theater during World War II.

Word History: Theories about the development of the theater in the West generally begin with Greek drama; this is etymologically appropriate as well as historically correct, since the words theory and theater are related through their Greek sources. The Greek ancestor of theater is theātron, "a place for seeing, especially for dramatic representation, theater." Theātron is derived from the verb theāsthai, "to gaze at, contemplate, view as spectators, especially in the theater," from theā, "a viewing." The Greek ancestor of theory is theōriā, which meant among other things "the sending of theōroi (state ambassadors sent to consult oracles or attend games)," "the act of being a spectator at the theater or games," "viewing," "contemplation by the mind," and "theory or speculation." The source of theōriā is theōros, "an envoy sent to consult an oracle, spectator," a compound of theā, "viewing," and -oros, "seeing." It is thus fitting to elaborate theories about culture while seeing a play in a theater.