The electromagnetic radiation that occurs in the Raman effect when the frequency of the scattered radiation is lower than the frequency of the unscattered radiation, i.e. energy is transferred from the photon to the scattering molecule. The spectral Raman lines corresponding to this type of radiation are called Stokes lines, with the set of Stokes lines being called the O-branch of the Raman spectrum.

Similarly, anti-Stokes radiation is the radiation when the frequency of the scattered electromagnetic radiation is higher than the frequency of the unscattered radiation, i.e. energy is transferred to the photon from the scattering molecule. The spectral Raman lines corresponding to this type of radiation are called anti-Stokes lines, with the set of anti-Stokes lines being called the S-branch of the Raman spectrum.

Stokes radiation is named after the British physicist Sir George Gabriel Stokes (1819–1903).