diopter vs dioptre

diopter and dioptre both are nouns.

Word	Noun	Adjective	Verb	Adverb
diopter	Yes	No	No	No
dioptre	Yes	No	No	No

As nouns, diopter and dioptre are synonyms defined as:

diopter and dioptre: a unit of measurement of the refractive power of a lens which is equal to the reciprocal of the focal length measured in meters; used by oculists

diopter (noun)	dioptre (noun)
a unit of measurement of the refractive power of a lens which is equal to the reciprocal of the focal length measured in meters; used by oculists	a unit of measurement of the refractive power of a lens which is equal to the reciprocal of the focal length measured in meters; used by oculists