fleur-de-lis vs fleur-de-lys

fleur-de-lis and fleur-de-lys both are nouns.

Word	Noun	Adjective	Verb	Adverb
fleur-de-lis	Yes	No	No	No
fleur-de-lys	Yes	No	No	No

As nouns, fleur-de-lis and fleur-de-lys are synonyms defined as:

fleur-de-lis and fleur-de-lys: (heraldry) charge consisting of a conventionalized representation of an iris

fleur-de-lis (noun)	fleur-de-lys (noun)
(heraldry) charge consisting of a conventionalized representation of an iris	(heraldry) charge consisting of a conventionalized representation of an iris
plants with sword-shaped leaves and erect stalks bearing bright-colored flowers composed of three petals and three drooping sepals