For a given transition from an excited state of a specified atom, the ratio of the number of excited atoms which emit a photon to the total number of excited atoms.
Source:
PAC, 1994, 66, 2513. 'Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)' on page 2526 (https://doi.org/10.1351/pac199466122513)
See also:
Orange Book, 2nd ed., p. 174 (https://media.iupac.org/publications/analytical_compendium/)