small letter vs lower-case letter

small letter and lower-case letter both are nouns.

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

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

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

Other synonyms of small letter include lowercase, minuscule.

small letter (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