Pronunciation: (dī-op'tur),[key] —n. 1. 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. Abbr.: D
2. 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. Also,esp. Brit.,di•op'tre.