1.NounA characteristic of an exponential function, represented by τ in the function:
"
y
(
t
)
=
y
o
e
−
t
/
τ
{\displaystyle y(t)\ =\ y_{o}e^{-t/\tau }\ }
."
2.NounThe 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.