( BrE ) ( NAmE di·opter ) / daɪˈɒptə(r); NAmE -ˈɑːp-/ noun
( physics ) a unit for measuring the power of a lens to refract light (=make it change direction)
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WORD ORIGIN
late 16th cent. (originally as diopter ): from French , from Latin dioptra , from Greek , from di- through + optos visible. The term was used in the early 17th cent. to denote an ancient form of theodolite; the current sense dates from the late 19th cent.