1. n. [Geophysics]

The principle that the path taken by a ray of light from one point to another is that which takes the minimum time (or the maximum time in select cases), named for its discoverer, French mathematician Pierre de Fermat (1601 to 1665). Snell's law and the laws of reflection and refraction follow from Fermat's principle. Fermat's principle also applies to seismic waves.

See related terms: least-time path,  seismic wave,  Snell's law,  tomography