hair vs pilus

hair and pilus both are nouns.

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

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

hair: a filamentous projection or process on an organism
pilus: hairlike structure especially on the surface of a cell or microorganism

Other hyponyms of hair include bristle, sensory hair, vibrissa, whisker, seta.

As nouns, hair and pilus are synonyms defined as:

hair and pilus: any of the cylindrical filaments characteristically growing from the epidermis of a mammal

hair (noun)	pilus (noun)
a filamentous projection or process on an organism	hairlike structure especially on the surface of a cell or microorganism
cloth woven from horsehair or camelhair; used for upholstery or stiffening in garments	any of the cylindrical filaments characteristically growing from the epidermis of a mammal
a covering for the body (or parts of it) consisting of a dense growth of threadlike structures (as on the human head); helps to prevent heat loss
any of the cylindrical filaments characteristically growing from the epidermis of a mammal
filamentous hairlike growth on a plant
a very small distance or space