The extent to which the output of the detector is directly proportional to the incident radiant power at a given wavelength and at constant irradiation geometry.
Source:
PAC, 1995, 67, 1745. 'Nomenclature, symbols, units and their usage in spectrochemical analysis-XI. Detection of radiation (IUPAC Recommendations 1995)' on page 1750 (https://doi.org/10.1351/pac199567101745)