A physical phenomenon during which the direction and intensity of a propagating wave is changed due to interaction with matter having structure dimensions in the order of the wavelength.

The term "matter" used in this definition is meant to represent crystalline as well as amorphous matter (e.g. gases or objects). Diffraction might be regarded as a specific type of elastic scattering. Diffracted waves are coherent and can interfere, forming diffraction patterns.The terms scattering and diffraction are used to describe the same phenomenon from different perspectives. From the view of a particle, people tend to speak of scattering, while from the view of waves and the related patterns, people speak of diffraction.Due to the wave-particle dualism, a beam of sub-atomic particles (e.g. electrons) can be diffracted.