diopter

noun

Etymology: From Latin dioptra, from Ancient Greek διόπτρα (dióptra).

1. A unit of measure of the power of a lens or mirror, equal to the reciprocal of its focal length in meters. Myopia is diagnosed and measured in diopters.
2. The dioptre adjustment mechanism of a pair of binoculars or of a camera viewfinder.
3. A lens element attached to the front of a photographic lens to shorten the lens's minimum focus distance; also called a "close-up filter".
4. Any lens system, such as a telescope.
5. A theodolite or similar surveyor’s angle measuring device.
6. An alidade.
7. A surgical speculum.
8. An instrument for drawing the skull by projections.