In modern usage, diopter or dioptre is a measure of the optical power of an optical system.
Diopter may also refer to: Historically, a dioptric lens Any lens system, such as a telescope A diopter sight A close-up lens used in photography A theodolite or similar surveyor's angle measuring device A dioptra, an old surveying device An alidade A surgical speculum An instrument for drawing the skull by projections
See also Dioptrice Dioptrics