A unit of measurement for the refractive power of a lens, equal to the reciprocal of the focal length in meters.
From Greek 'dioptron' (optical instrument used to see through). The term was formalized in the 19th century as optometry developed into a precise science with standardized measurements.
When an eye doctor says you need a +2 diopter prescription, they're using a unit that didn't exist before people started treating vision as a mathematical problem rather than just 'you see bad.'
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