di·op·ter 
(dī-ŏptər)
(dī-ŏptər)n.
A unit of measurement of the refractive power of lenses equal to the reciprocal of the focal length measured in meters.
[Obsolete diopter, an instrument for measuring angles, from Latin dioptra, from Greek dioptrā : dia-, dia- + optos, visible; see okw- in the Appendix of Indo-European roots.]
di·optral (-trəl) adj.