filum vs pilus

filum and pilus both are nouns.

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

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

filum: a threadlike structure (as a chainlike series of cells)
pilus: any of the cylindrical filaments characteristically growing from the epidermis of a mammal

Other hyponyms of filum include hair.

filum (noun)	pilus (noun)
a threadlike structure (as a chainlike series of cells)	hairlike structure especially on the surface of a cell or microorganism
	any of the cylindrical filaments characteristically growing from the epidermis of a mammal