Meaning of principal axis

prin'cipal ax'is

Pronunciation: [key]
  1. a line passing through the center of the surface of a lens or spherical mirror and through the centers of curvature of all segments of the lens or mirror.
  2. one of three mutually perpendicular axes of a body about which the moment of inertia is maximum or minimum.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.