Definition from Wiktionary, the free dictionary
See also Theater
Alternative forms 
- theatre (especially UK)
From Middle English theater, theatre, from Old French theatre, from Latin theatrum, from Ancient Greek θέατρον (théatron, “a place for viewing”), from θεάομαι (theáomai, “to see", "to watch", "to observe”).
theater (plural theaters)
- 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.
- A region where a particular action takes place; a specific field of action, usually with reference to war.
- His grandfather was in the Pacific theater during the war.
- A lecture theatre.
- (medicine) An operating theatre or locale for human experimentation.
- This man is about to die, get him into theater at once!
- (US) A cinema.
- We sat in the back row of the theater and threw popcorn at the screen.
- Drama or performance as a profession or artform.
- I worked in the theater for twenty-five years.
Usage notes 
- The spelling theatre is the main spelling in Commonwealth English, with theater being rare.
- In United States English, theater accounts for about 80 percent of usage in the major corpus of usage, COCA.
place or building
medicine: operating theatre — see operating theatre
drama or performance as a profession or artform
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