The rotating-wave approximation is an approximation used in atom optics and magnetic resonance. In this approximation, terms in a Hamiltonian that oscillate rapidly are neglected. This is a valid approximation when the applied electromagnetic radiation is near resonance with an atomic transition, and the intensity is low. Explicitly, terms in the Hamiltonians that oscillate with frequencies ω ω L + ω ω 0 are neglected, while terms that oscillate with frequencies ω ω L − − ω ω 0 are kept, where ω ω L is the light frequency, and ω ω 0 is a transition frequency.