time constant

Short definition from Wiktionary

Noun

1. 1
   A characteristic of an exponential function, represented by τ in the function
2. 2
   The time in which a physical system's response to a step-wise change in an external variable reaches approximately 63% of its final (asymptotic) value.
3. 3
   For radioactivity: the decay constant.
4. 4
   Alternative form of time constant.

Adjective

1. 1
   Of or pertaining to a time constant.