yus vs letter of the alphabet

yus and letter of the alphabet both are nouns.

Word	Noun	Adjective	Verb	Adverb
yus	Yes	No	No	No
letter of the alphabet	Yes	No	No	No

As nouns, letter of the alphabet is a hypernym of yus; that is, letter of the alphabet is a word with a broader meaning than yus:

yus: a Common Slavonic nasal vowel in the early Cyrillic and Glagolitic alphabets.
letter of the alphabet: the conventional characters of the alphabet used to represent speech

Other hypernyms of yus include alphabetic character, letter.

yus (noun)	letter of the alphabet (noun)
a Common Slavonic nasal vowel in the early Cyrillic and Glagolitic alphabets.	the conventional characters of the alphabet used to represent speech