Scattering of x-rays and gamma radiation by matter in which the frequency of the scattered radiation is measurably less than that of the incident radiation (inelastic scattering). So named because Arthur Compton was the first to explain the observed frequency shift by applying the laws of energy and momentum conservation to scattering of a photon by a free electron. Compton's experimental and theoretical investigations established the validity of the quantum theory of radiation, showing that photons possess momentum (and hence can exert radiation pressure) as well as energy.