capital letter vs grapheme

capital letter and grapheme both are nouns.

Word	Noun	Adjective	Verb	Adverb
capital letter	Yes	No	No	No
grapheme	Yes	No	No	No

As nouns, grapheme is a hypernym of capital letter; that is, grapheme is a word with a broader meaning than capital letter:

capital letter: one of the large alphabetic characters used as the first letter in writing or printing proper names and sometimes for emphasis
grapheme: a written symbol that is used to represent speech

Other hypernyms of capital letter include character, graph, graphic symbol.

capital letter (noun)	grapheme (noun)
one of the large alphabetic characters used as the first letter in writing or printing proper names and sometimes for emphasis	a written symbol that is used to represent speech