That mode of emission which occurs even in the absence of a perturbing external electromagnetic field. The transition between states, n and m, is governed by the einstein coefficient of spontaneous emission, Am.
See also: stimulated emission
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2277 (https://doi.org/10.1351/pac199668122223)