lowercase vs small letter

lowercase and small letter both are nouns.

Word	Noun	Adjective	Verb	Adverb
lowercase	Yes	Yes	No	No
small letter	Yes	No	No	No

As nouns, lowercase and small letter are synonyms defined as:

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

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

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