- Optics. a unit of measure of the refractive power of a lens, having the dimension of the reciprocal of length and a unit equal to the reciprocal of one meter. Abbreviation: D
- an instrument, invented by Hipparchus, to measure the apparent diameter of the sun or moon or to estimate the size or elevation of distant objects.
- A unit of measurement of the refractive power of lenses equal to the reciprocal of the focal length measured in meters.