diopter

from WordNet (r) 3.0 (2006)

diopter
    n 1: a unit of measurement of the refractive power of a lens
         which is equal to the reciprocal of the focal length
         measured in meters; used by oculists [syn: {diopter},
         {dioptre}]

from The Collaborative International Dictionary of English v.0.48

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.
   [1913 Webster]

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