Fermat's principle

Etymology
First proposed by French mathematician in 1662.

Proper noun

 * 1)  The principle, which links  (or "ray optics") with, that the path traversed by a ray between two given points is:  the one that takes the least time, or  one that takes a time that is "stationary" with respect to variations of the path (so that, loosely speaking, a small change in the ray path entails a very small change in the traversal time).
 * 2) * 1976, Donald E. Tilley, Walter Thumm, Physics for College Students, Cummings Publishing Company, page 558,
 * students wishing to pursue this branch of physics further will find that refraction too can be explained in terms of Fermat's principle.
 * 1) * 1976, Donald E. Tilley, Walter Thumm, Physics for College Students, Cummings Publishing Company, page 558,
 * students wishing to pursue this branch of physics further will find that refraction too can be explained in terms of Fermat's principle.

Translations

 * French: principe de Fermat
 * German: Fermatsches Prinzip
 * Italian: