the point where a principal plane intersects the axis.
Origin of principal point
First recorded in 1695–1705
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
One of the two points in an optical system where the axis is cut by the two principal planes. Lines drawn from these to corresponding points on the object and the image will be parallel.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.