Roman vs Roman Empire

Roman and Roman Empire both are nouns.

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

Roman (noun)	Roman Empire (noun)
a resident of modern Rome	an empire established by Augustus in 27 BC and divided in AD 395 into the Western Roman Empire and the eastern or Byzantine Empire; at its peak lands in Europe and Africa and Asia were ruled by ancient Rome
an inhabitant of the ancient Roman Empire

Roman (adjective)	Roman Empire (adjective)
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