The time required for the luminescence intensity to decay from some initial value to 1/e of that value (e = 2.718 ...). Lifetimes can be measured by phase fluorimetry (phosphorimetry) where the phase shift between the sinusoidally modulated exciting light and the emitted light is measured.
Source:
PAC, 1984, 56, 231. 'Nomenclature, symbols, units and their usage in spectrochemical analysis-Part VI: molecular luminescence spectroscopy' on page 242 (https://doi.org/10.1351/pac198456020231)