capital letter vs upper-case letter

capital letter and upper-case letter both are nouns.

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

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

capital letter and upper-case 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 capital letter include capital, majuscule, uppercase.

capital letter (noun)	upper-case 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