codfish vs salt cod

codfish and salt cod both are nouns.

Word	Noun	Adjective	Verb	Adverb
codfish	Yes	No	No	No
salt cod	Yes	No	No	No

As nouns, salt cod is a hyponym of codfish; that is, salt cod is a word with a more specific, narrower meaning than codfish:

codfish: lean white flesh of important North Atlantic food fish; usually baked or poached
salt cod: codfish preserved in salt; must be desalted and flaked by soaking in water and pounding; used in e.g. codfish cakes

codfish (noun)	salt cod (noun)
major food fish of Arctic and cold-temperate waters	codfish preserved in salt; must be desalted and flaked by soaking in water and pounding; used in e.g. codfish cakes
lean white flesh of important North Atlantic food fish; usually baked or poached