A physical phenomenon during which the trajectory of a particle is changed due to interaction with matter.

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. 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.