hard roe vs caviar

hard roe and caviar both are nouns.

Word	Noun	Adjective	Verb	Adverb
hard roe	Yes	No	No	No
caviar	Yes	No	No	No

As nouns, caviar is a hyponym of hard roe; that is, caviar is a word with a more specific, narrower meaning than hard roe:

hard roe: fish eggs or egg-filled ovary; having a grainy texture
caviar: salted roe of sturgeon or other large fish; usually served as an hors d'oeuvre

Other hyponyms of hard roe include coral, caviare, shad roe.

hard roe (noun)	caviar (noun)
fish eggs or egg-filled ovary; having a grainy texture	salted roe of sturgeon or other large fish; usually served as an hors d'oeuvre