1. theater (a place or building, consisting of a stage and seating, in which an audience gathers to watch plays, musical performances, public ceremonies, and so on) (countable)

2. theater; drama (drama or performance as a profession or art form)

3. theater (a region where a particular action takes place; a specific field of action, usually with reference to war)

4. theatrics; drama; melodrama (exaggerated reaction to life events)

5. movie theater; cinema (US, chiefly in names)