1) A unit of optical power that expresses the refractive power of a lens or a mirror. For a lens, it is equal to the reciprocal of the focal length, in meters. For example, a 5 diopter lens brings parallel rays of light shining on it to a focus 1/5 of a meter away from its center of curvature. 2) A prism diopter (∆) is a measure of prismatic deviation equal to a deflection of 1 cm at a distance of 1 m.
From L. dioptra, from Gk. dioptra : dia- + optos, "visible." A unit of measurement of the refractive power of lenses equal to the reciprocal of the focal length measured in meters.