Fermat's principle
noun
Optics.
the law that the path taken by a ray of light in going from one point to another point will be the path that requires the least time.
Origin of Fermat's principle
First recorded in 1885–90; named after P. de
Fermat
British Dictionary definitions for fermat's principle
Fermat's principle
noun
physics
the principle that a ray of light passes from one point to another in such a way that the time taken is a minimum