Meaning of principal plane

prin'cipal plane'

Pronunciation: [key]
— Optics. Optics.
  1. a plane that is perpendicular to the axis of a lens, mirror, or other optical system and at which rays diverging from a focal point are deviated parallel to the axis or at which rays parallel to the axis are deviated to converge to a focal point.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.