Meaning of numerical aperture

numer'ical ap'erture

Pronunciation: [key]
— Micros. Micros.
  1. a measure of the resolving power of a microscope, equal to the index of refraction of the medium in which the object is placed multiplied by the sine of the angle made with the axis by the most oblique ray entering the instrument, the resolving power increasing as the product increases. Abbr.: N.A.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.