Pronunciation: (dī-op'tur), [key]
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.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.