Source : Webster's Revised Unabridged Dictionary (1913)

Diopter \Di*op"ter\, Dioptra \Di*op"tra\, n. [L. dioptra, fr.
   Gr. ?. See 2d {Dioptric}.]
   An optical instrument, invented by Hipparchus, for taking
   altitudes, leveling, etc.

Source : WordNet®

diopter
     n : a unit of measurement of the refractive power of a lens
         which is equal to the reciprocal of the focal length
         measured in meters