Huygens' principle is a fundamental concept in wave theory, particularly in optics, that explains how waves propagate through a medium. It was formulated by Dutch scientist Christiaan Huygens in the 17th century. Huygens' principle states:

"Every point on a wavefront can be considered as a source of secondary wavelets that spread out in all directions with the same speed as the wave itself. The new wavefront at any later time is the envelope of these secondary wavelets."

In simpler terms, Huygens' principle suggests that each point on a wavefront acts as a source of secondary spherical wavelets. The sum of these wavelets creates a new wavefront at a later time, determining the direction of wave propagation.