upper-case letter vs capital letter

upper-case letter and capital letter both are nouns.

Word	Noun	Adjective	Verb	Adverb
upper-case letter	Yes	No	No	No
capital letter	Yes	No	No	No

As nouns, upper-case letter and capital letter are synonyms defined as:

upper-case letter and capital letter: one of the large alphabetic characters used as the first letter in writing or printing proper names and sometimes for emphasis

Other synonyms of upper-case letter include capital, majuscule, uppercase.

upper-case letter (noun)	capital letter (noun)
one of the large alphabetic characters used as the first letter in writing or printing proper names and sometimes for emphasis	one of the large alphabetic characters used as the first letter in writing or printing proper names and sometimes for emphasis