fleetingness vs transience

fleetingness and transience both are nouns.

Word	Noun	Adjective	Verb	Adverb
fleetingness	Yes	No	No	No
transience	Yes	No	No	No

As nouns, transience is a hypernym of fleetingness; that is, transience is a word with a broader meaning than fleetingness:

fleetingness: the property of lasting for a very short time
transience: an impermanence that suggests the inevitability of ending or dying

Other hypernyms of fleetingness include transiency, transitoriness.

fleetingness (noun)	transience (noun)
the property of lasting for a very short time	an impermanence that suggests the inevitability of ending or dying
	the attribute of being brief or fleeting