lowercase vs lower-case letter

lowercase and lower-case letter both are nouns.

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

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

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

Other synonyms of lowercase include minuscule, small letter.

lowercase (noun)	lower-case letter (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

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