1. The time taken for the current in a circuit having a steady emf to reach a definite fraction of its final value after the circuit is closed. The fraction is (1–1/e)=0.632. 2. The time taken for the current to decay to 1/e=0.368 of its value after the emf is removed. Also called decay constant or relaxation time. 3. AGC time constant (q.v.). 4. A time over which readings are averaged to remove statistical fluctuations, as with nuclear-log readings.