starfish vs sea star

starfish and sea star both are nouns.

Word	Noun	Adjective	Verb	Adverb
starfish	Yes	No	No	No
sea star	Yes	No	No	No

As nouns, starfish and sea star are synonyms defined as:

starfish and sea star: echinoderms characterized by five arms extending from a central disk

starfish (noun)	sea star (noun)
echinoderms characterized by five arms extending from a central disk	echinoderms characterized by five arms extending from a central disk