brilliancy vs brightness

brilliancy and brightness both are nouns.

Word	Noun	Adjective	Verb	Adverb
brilliancy	Yes	No	No	No
brightness	Yes	No	No	No

As nouns, brightness is a hypernym of brilliancy; that is, brightness is a word with a broader meaning than brilliancy:

brilliancy: a quality that outshines the usual
brightness: the location of a visual perception along a continuum from black to white

brilliancy (noun)	brightness (noun)
a quality that outshines the usual	the location of a visual perception along a continuum from black to white
	the quality of being luminous; emitting or reflecting light
	intelligence as manifested in being quick and witty