A parametric process is an optical process in which light interacts with matter in such a way as to leave the quantum state of the material unchanged. As a direct consequence of this there can be no net transfer of energy, momentum, or angular momentum between the optical field and the physical system. In contrast a non-parametric process is a process in which any part of the quantum state of the system changes.See Section Parametric versus Nonparametric Processes, Nonlinear Optics by Robert W. Boyd (3rd ed.), pp. 13-15.
