(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 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.