ingrown hair vs pilus

ingrown hair and pilus both are nouns.

Word	Noun	Adjective	Verb	Adverb
ingrown hair	Yes	No	No	No
pilus	Yes	No	No	No

As nouns, pilus is a hypernym of ingrown hair; that is, pilus is a word with a broader meaning than ingrown hair:

ingrown hair: a hair that does not emerge from the follicle but remains embedded in the skin (usually causing inflammation)
pilus: any of the cylindrical filaments characteristically growing from the epidermis of a mammal

Other hypernyms of ingrown hair include hair.

ingrown hair (noun)	pilus (noun)
a hair that does not emerge from the follicle but remains embedded in the skin (usually causing inflammation)	hairlike structure especially on the surface of a cell or microorganism
	any of the cylindrical filaments characteristically growing from the epidermis of a mammal