The story
First proposed by French mathematician Pierre de Fermat in 1662.
Etymology from Wiktionary, CC BY-SA 4.0.
Fermat's principle
Today it means: The principle, which links geometrical optics (or "ray optics") with wave optics, that the path traversed by a ray between two given points is: (in the original "strong" formulation) the one that takes the least time, or (in a weaker but more general formulation) 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). Full definition →