arena theater vs theater in the round

arena theater and theater in the round both are nouns.

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

As nouns, arena theater and theater in the round are synonyms defined as:

arena theater and theater in the round: a theater arranged with seats around at least three sides of the stage

arena theater (noun)	theater in the round (noun)
a theater arranged with seats around at least three sides of the stage	a theater arranged with seats around at least three sides of the stage