eight vs digit

eight and digit both are nouns.

Word	Noun	Adjective	Verb	Adverb
eight	Yes	Yes	No	No
digit	Yes	No	No	No

As nouns, digit is a hypernym of eight; that is, digit is a word with a broader meaning than eight:

Other hypernyms of eight include figure.

eight (noun)	digit (noun)
one of four playing cards in a deck with eight pips on the face	a finger or toe in human beings or corresponding body part in other vertebrates
the cardinal number that is the sum of seven and one	one of the elements that collectively form a system of numeration
	the length of breadth of a finger used as a linear measure

eight (adjective)	digit (adjective)
being one more than seven