pilus vs ingrown hair

pilus and ingrown hair both are nouns.

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

As nouns, ingrown hair is a hyponym of pilus; that is, ingrown hair is a word with a more specific, narrower meaning than pilus:

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

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