Response Time is the time given by the inertia of a component and is reflected in hysteresis. These can be mechanical inertias, mass inertias, or stray capacitances and inductances that counteract a faster response.
The response time can be related to thermal, emission, magnetic events or to electrical pulses, currents or voltages. It is the time that elapses between the start of the action, for example the activation of a light emitting diode, and the initiation of the response, in this case the lighting of the light emitting diode. The response time is given as the 90% value.