labium vs labia majora

labium and labia majora both are nouns.

Word	Noun	Adjective	Verb	Adverb
labium	Yes	No	No	No
labia majora	Yes	No	No	No

As nouns, labia majora is a hyponym of labium; that is, labia majora is a word with a more specific, narrower meaning than labium:

labium: a liplike structure that bounds a bodily orifice (especially any of the four labiate folds of a woman's vulva)
labia majora: the two outer folds of the vulva

Other hyponyms of labium include labia minora.

labium (noun)	labia majora (noun)
a liplike structure that bounds a bodily orifice (especially any of the four labiate folds of a woman's vulva)	the two outer folds of the vulva