time constant, τ c

If the output of a detector changes exponentially with time, the time required for it to change from its initial value by the fraction 1 − exp ( − t / τ c ) (for t = τ c) of the final value, is called the time constant.
PAC, 1995, 67, 1745 (Nomenclature, symbols, units and their usage in spectrochemical analysis-XI. Detection of radiation (IUPAC Recommendations 1995)) on page 1751