lower-case letter vs small letter

lower-case letter and small letter both are nouns.

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

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

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

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

lower-case letter (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