time constant

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mathematics A characteristic of an exponential function, represented by τ in the function:
• noun physics 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the ratio of the inductance of a circuit in henries to its resistance in ohms
• noun (electronics) the time required for the current or voltage in a circuit to rise or fall exponentially through approximately 63 per cent of its amplitude