theater in the round vs theatre

theater in the round and theatre both are nouns.

Word	Noun	Adjective	Verb	Adverb
theater in the round	Yes	No	No	No
theatre	Yes	No	No	No

As nouns, theatre is a hypernym of theater in the round; that is, theatre is a word with a broader meaning than theater in the round:

theater in the round: a theater arranged with seats around at least three sides of the stage
theatre: a building where theatrical performances or motion-picture shows can be presented

Other hypernyms of theater in the round include house, theater.

theater in the round (noun)	theatre (noun)
a theater arranged with seats around at least three sides of the stage	a building where theatrical performances or motion-picture shows can be presented
	the art of writing and producing plays
	a region in which active military operations are in progress