serum vs milk whey

serum and milk whey both are nouns.

Word	Noun	Adjective	Verb	Adverb
serum	Yes	No	No	No
milk whey	Yes	No	No	No

As nouns, milk whey is a hyponym of serum; that is, milk whey is a word with a more specific, narrower meaning than serum:

serum: an amber, watery fluid, rich in proteins, that separates out when blood coagulates
milk whey: the serum or watery part of milk that is separated from the curd in making cheese

Other hyponyms of serum include antiserum, whey.

serum (noun)	milk whey (noun)
an amber, watery fluid, rich in proteins, that separates out when blood coagulates	the serum or watery part of milk that is separated from the curd in making cheese