1. The value t in an exponential response term. For the output of a first order system forced by a step or an impulse, t is the time required to complete 63.2% of the total rise or decay. In higher order systems, there is a time constant for each of the first order components of the process. 2. The length of time required for the output of a transducer to rise to 63% of its final value as a result of a step change of input.