natural broadening

of a spectral line
Broadening which has its origin in the finite optical lifetime of one or both levels involved in the transition.
Orange Book, 2nd ed., p. 121 (
PAC, 1985, 57, 1453. 'Nomenclature, symbols, units and their usage in spectrochemical analysis - V: Radiation sources (Recommendations 1985)' on page 1463 (