General term for certain cases of light emission. If excitation is by light, one speaks of photoluminescence. This is the case when molecules are excited by the absorption of optical radiation energy and then emit energy when they quiet down to lower energy levels. The wavelength of the secondary radiation is equal to or greater than that of the primary radiation (Stokes's Law).
Fluorescence: luminescence that persists only instantaneously after excitation.
Phosphorescence: luminescence that persists for an appreciable time after excitation.