Roman alphabet vs alphabet

Roman alphabet and alphabet both are nouns.

Word	Noun	Adjective	Verb	Adverb
Roman alphabet	Yes	No	No	No
alphabet	Yes	No	No	No

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

Roman alphabet: the alphabet evolved by the ancient Romans which serves for writing most of the languages of western Europe
alphabet: a character set that includes letters and is used to write a language

Other hypernyms of Roman alphabet include ABC's, ABC, ABCs, bicameral script.

Roman alphabet (noun)	alphabet (noun)
the alphabet evolved by the ancient Romans which serves for writing most of the languages of western Europe	the elementary stages of any subject (usually plural)
	a character set that includes letters and is used to write a language