halibut vs flatfish

halibut and flatfish both are nouns.

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

As nouns, flatfish is a hypernym of halibut; that is, flatfish is a word with a broader meaning than halibut:

halibut: marine food fish of the northern Atlantic or northern Pacific; the largest flatfish and one of the largest teleost fishes
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

As nouns, flatfish is a hypernym of halibut; that is, flatfish is a word with a broader meaning than halibut:

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

halibut (noun)	flatfish (noun)
marine food fish of the northern Atlantic or northern Pacific; the largest flatfish and one of the largest teleost fishes	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
lean flesh of very large flatfish of Atlantic or Pacific	sweet lean whitish flesh of any of numerous thin-bodied fish; usually served as thin fillets