Main Entry: di·op·ter

Pronunciation: \dī-ˈäp-tər, ˈdī-ˌäp-\

Function: noun

Etymology: diopter (an optical instrument), from Middle French dioptre, from Latin dioptra, from Greek, from dia- + opsesthai to be going to see — more at optic

Date: circa 1864

: a unit of measurement of the refractive power of lenses equal to the reciprocal of the focal length in meters