mammary gland vs udder

mammary gland and udder both are nouns.

Word	Noun	Adjective	Verb	Adverb
mammary gland	Yes	No	No	No
udder	Yes	No	No	No

As nouns, udder is a hyponym of mammary gland; that is, udder is a word with a more specific, narrower meaning than mammary gland:

Other hyponyms of mammary gland include dug, bag, boob, bosom, breast, knocker, tit, titty.

mammary gland (noun)	udder (noun)
milk-secreting organ of female mammals	mammary gland of bovids (cows and sheep and goats)