whole number vs digit

whole number and digit both are nouns.

Word	Noun	Adjective	Verb	Adverb
whole number	Yes	No	No	No
digit	Yes	No	No	No

As nouns, digit is a hyponym of whole number; that is, digit is a word with a more specific, narrower meaning than whole number:

whole number (noun)	digit (noun)
any of the natural numbers (positive or negative) or zero	a finger or toe in human beings or corresponding body part in other vertebrates
	one of the elements that collectively form a system of numeration
	the length of breadth of a finger used as a linear measure