Italian vs Roman

Italian and Roman both are nouns.

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

As nouns, Roman is a hyponym of Italian; that is, Roman is a word with a more specific, narrower meaning than Italian:

Italian (noun)	Roman (noun)
the Romance language spoken in Italy	a resident of modern Rome
a native or inhabitant of Italy	an inhabitant of the ancient Roman Empire

Italian (adjective)	Roman (adjective)
of or pertaining to or characteristic of Italy or its people or culture or language	of or relating to or supporting Romanism
	of or relating to or derived from Rome (especially ancient Rome)
	characteristic of the modern type that most directly represents the type used in ancient Roman inscriptions