flatfish vs halibut

flatfish and halibut both are nouns.

Word	Noun	Adjective	Verb	Adverb
flatfish	Yes	No	No	No
halibut	Yes	No	No	No

As nouns, halibut is a hyponym of flatfish; that is, halibut is a word with a more specific, narrower meaning than flatfish:

flatfish: any of several families of fishes having flattened bodies that swim along the sea floor on one side of the body with both eyes on the upper side
halibut: marine food fish of the northern Atlantic or northern Pacific; the largest flatfish and one of the largest teleost fishes

As nouns, halibut is a hyponym of flatfish; that is, halibut is a word with a more specific, narrower meaning than flatfish:

flatfish: sweet lean whitish flesh of any of numerous thin-bodied fish; usually served as thin fillets
halibut: lean flesh of very large flatfish of Atlantic or Pacific

Other hyponyms of flatfish include flounder, fillet of sole, sole.

flatfish (noun)	halibut (noun)
any of several families of fishes having flattened bodies that swim along the sea floor on one side of the body with both eyes on the upper side	marine food fish of the northern Atlantic or northern Pacific; the largest flatfish and one of the largest teleost fishes
sweet lean whitish flesh of any of numerous thin-bodied fish; usually served as thin fillets	lean flesh of very large flatfish of Atlantic or Pacific