From AMS Glossary
A change of direction and possibly amplitude of an electromagnetic, acoustic, or any other wave propagating in a material medium, as a consequence of spatial variation in the properties of the medium.
In specular (mirrorlike) reflection, the spatial variation is abrupt (on the scale of the wavelength), as at an interface between water and air. Specular reflection is described by the law of reflection, according to which incident and reflected waves lie in the plane of incidence, defined by the normal to the interface and the direction of the incident wave, and make the same angle of incidence with this normal. Specular reflection is distinguished from refraction in that the direction of propagation of the reflected wave has a component opposite the direction of the incident wave. Although the law of (specular) reflection is often a good approximation, it is not exact: Diffuse reflection in directions not accounted for by the law of reflection always accompanies specular reflection because matter is not homogeneous on all scales. Light reflected by a cloud illuminated by sunlight is an example of diffuse reflection. Reflection may also refer to the change of direction of a beam of particles, in the broadest sense of this term.