semicircle vs hemicycle

semicircle and hemicycle both are nouns.

Word	Noun	Adjective	Verb	Adverb
semicircle	Yes	No	No	No
hemicycle	Yes	No	No	No

As nouns, semicircle and hemicycle are synonyms defined as:

semicircle and hemicycle: a plane figure with the shape of half a circle

semicircle (noun)	hemicycle (noun)
a plane figure with the shape of half a circle	a plane figure with the shape of half a circle