From AMS Glossary
The process by which the direction of radiation is changed so that it spreads into the geometric shadow zone of an opaque or refractive object that lies in a radiation field.
Diffraction is an optical "edge effect," differing only in degree from scattering. Diffraction becomes more evident when dealing with particles similar to, or larger than, the wavelength of the radiation. In meteorological optics, important diffraction phenomena include the aureole, Bishop's ring, corona, iridescent clouds, etc. The principle of diffraction may also be applied to the propagation of water surface waves, as into the sheltered region formed by a barrier.