h vs alphabetic character

h and alphabetic character both are nouns.

Word	Noun	Adjective	Verb	Adverb
h	Yes	No	No	No
alphabetic character	Yes	No	No	No

As nouns, alphabetic character is a hypernym of h; that is, alphabetic character is a word with a broader meaning than h:

h: the 8th letter of the Roman alphabet
alphabetic character: the conventional characters of the alphabet used to represent speech

Other hypernyms of h include letter, letter of the alphabet.

h (noun)	alphabetic character (noun)
the 8th letter of the Roman alphabet	the conventional characters of the alphabet used to represent speech
the constant of proportionality relating the energy of a photon to its frequency; approximately 6.626 x 10^-34 joule-second