(1) The time in seconds for a measuring instrument to register a 63% change from a former level of response toward a new level of response.

(2) In [[nuclear log|nuclear logging]], because of the random nature of [[nuclear]] emissions, the [[detector]] output is averaged over a selected time interval in order to record the representative [[radiation]] level of the environment. Instrument response will thus adjust gradually to environmental changes depending on the length of the averaging time (i.e., time constant). See also [[lag]].