[ dʌɪ'ɒptə ]
( US diopter )
■ noun a unit of refractive power, which is equal to the reciprocal of the focal length (in metres) of a given lens.
Origin
C16 (orig. denoting an alidade, later a form of theodolite): from Fr., from L. dioptra , from Gk, from di- 'through' + optos 'visible'.