Transition time

The transition time $t_t\,$ is the time a dynamical system needs to switch between two different stable states, when responding to a stable input signal. In a logic circuit (a discrete-time dynamical system whose state is representable as a boolean-valued vector function of time) undergoing a change of state, it identifies the rise time or the fall time of the output voltage. It is therefore correct to speak of two types of transition times

$t_{tH}\,$ : transition time Low-to-High. It is the rise time of a logic gate's output voltage
$t_{tL}\,$ : transition time High-to-Low. It is the fall time of a logic gate's output voltage