For a measurement, the time during which a radiation measuring assembly is capable of processing events occurring in the radiation detector. It equals the clock time minus the integrated resolving or dead time (to be distinguished from 'life time').
Source:
PAC, 1994, 66, 2513. 'Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)' on page 2522 (https://doi.org/10.1351/pac199466122513)