In general:
The time between the input signal and the increase of the output signal to a given value, measured under certain conditions.
Of a signal element:
Time which passes after reaching a certain input signal (increasing or decreasing) until the output signal has reached a certain percentage of its maximum value.
Of a solenoid-actuated valve:
Time which passes between the closing (or opening) of the electrical contact before the outlet pressure of the valve has reached a certain percentage of its maximum value and the flow rate is equal to the prescribed value.
Of a pressure-actuated valve:
Time which passes after reaching a certain control pressure (increasing or decreasing) until the outlet pressure has reached a given percentage of its maximum value and the flow rate is equal to the prescribed value (CETOP RP 100).