diopter
noun
di·op·ter dī-ˈäp-tər 
ˈdī-ˌäp-
: a unit of measurement of the refractive power of lenses equal to the reciprocal of the focal length in meters
Example Sentences
Recent Examples on the Web Monoyer invented the diopter, a unit of measurement that determines the distance someone must be from a text to read it. 
Brett Molina, USA TODAY, 9 May 2017 Perhaps the glasses’ increasing diopters told a narrative. The Washington Post, The Mercury News, 13 Mar. 2017
Brett Molina, USA TODAY, 9 May 2017 Perhaps the glasses’ increasing diopters told a narrative. The Washington Post, The Mercury News, 13 Mar. 2017 Word History
Etymology
diopter (an optical instrument), from Middle French dioptre, from Latin dioptra, from Greek, from dia- + opsesthai to be going to see — more at optic
First Known Use
circa 1864, in the meaning defined above
Medical Definition
diopter
noun
di·op·ter
variants or chiefly British dioptre
dī-ˈäp-tər, ˈdī-ˌäp-
: a unit of measurement of the refractive power of a lens equal to the reciprocal of the focal length in meters