The Collaborative International Dictionary of English v.0.48:

Dioptre \Di*op"tre\, n. [F. See 2d Dioptric.] (Optics)
   A unit employed by oculists in numbering glasses according to
   the metric system; a refractive power equal to that of a
   glass whose principal focal distance is one meter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dioptre
    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]