the·a·ter or the·a·tre 
(thēə-tər)
(thēə-tər)n.
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.
a. Dramatic literature or its performance; drama: the theater of Shakespeare and Marlowe.
b. The milieu of actors and playwrights.
4.
a. The quality or effectiveness of a theatrical production: good theater; awful theater.
b. 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.
[Middle English theatre, from Old French, from Latin theātrum, from Greek theātron, from theāsthai, to watch, from theā, a viewing.]