Meaning of curvature of field

cur'vature of field'

Pronunciation: [key]
— Optics. Optics.
  1. a monochromatic aberration of a lens or other optical system in which the focal surface is curved, the refracted image of an object oriented perpendicular to the axis of the lens lying on a curved surface rather than in a plane perpendicular to the axis.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.