greatness vs illustriousness

greatness and illustriousness both are nouns.

Word	Noun	Adjective	Verb	Adverb
greatness	Yes	No	No	No
illustriousness	Yes	No	No	No

As nouns, greatness and illustriousness are synonyms defined as:

greatness and illustriousness: the property possessed by something or someone of outstanding importance or eminence

greatness (noun)	illustriousness (noun)
the property possessed by something or someone of outstanding importance or eminence	the property possessed by something or someone of outstanding importance or eminence
unusual largeness in size or extent or number