Also called the delay time. It is the time, which the output signal of a 1st order delay element requires to reach 63% of the end value. After 3 · T 95 % and at 5 · T almost 100 % of the end value (Figure Z 3).
The term time constant is –– not completely correctly –– used if systems (e.g. control circuits) can be assumed to be 1st order systems (e.g. if the natural frequency is somewhat dampened and the valve behaviour in the subsystem does not play a role) or in the case of comparable elements with an increase according to an exponential function. The time constant results as the inverse of the loop gain:
Figure Z 3: Transfer function of a PT1 element