Optics. 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.
Physics. one of three mutually perpendicular axes of a body about which the moment of inertia is maximum or minimum.
Origin of principal axis
First recorded in 1875–80
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
the line passing through the optical centre and centres of curvature of the faces of a lens or a curved mirror
any of three mutually perpendicular axes about which the moment of inertia of a body is maximum
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012