roman vs roman letters

roman and roman letters both are nouns.

Word	Noun	Adjective	Verb	Adverb
roman	Yes	No	No	No
roman letters	Yes	No	No	No

As nouns, roman and roman letters are synonyms defined as:

Other synonyms of roman include roman print, roman type.

roman (noun)	roman letters (noun)
a typeface used in ancient Roman inscriptions	a typeface used in ancient Roman inscriptions