fleetingness vs transiency

fleetingness and transiency both are nouns.

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

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

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

Other hypernyms of fleetingness include transience, transitoriness.

fleetingness (noun)	transiency (noun)
the property of lasting for a very short time	an impermanence that suggests the inevitability of ending or dying