sea star vs starfish

sea star and starfish both are nouns.

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

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

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

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