lower-case letter vs lowercase

lower-case letter and lowercase both are nouns.

Word	Noun	Adjective	Verb	Adverb
lower-case letter	Yes	No	No	No
lowercase	Yes	Yes	No	No

As nouns, lower-case letter and lowercase are synonyms defined as:

lower-case letter and lowercase: the characters that were once kept in bottom half of a compositor's type case

Other synonyms of lower-case letter include minuscule, small letter.

lower-case letter (noun)	lowercase (noun)
the characters that were once kept in bottom half of a compositor's type case	the characters that were once kept in bottom half of a compositor's type case

lower-case letter (adjective)	lowercase (adjective)
	relating to small (not capitalized) letters that were kept in the lower half of a compositor's type case