decimal digit vs digit

decimal digit and digit both are nouns.

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

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

Other hypernyms of decimal digit include figure.

decimal digit (noun)	digit (noun)
a digit from 0 to 9 in decimal notation	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