Ability of an amplifier or loudspeaker to follow accurately abrupt changes, such as the sudden burst of sound generated by an instrument. Good transient response is vital to "clear" or "crisp" overall sound.
The short-term response of an instrument caused by a change of status of the instrument's environment. For example, the switching of a power supply on and off will send very short-term power spikes that can be detected by a volt meter with sufficiently rapid response time.
The response of a circuit to a sudden change in an input or output quantity. In power supplies, this is the excursion of the output voltage and the time it takes to recover from a step change in the output load or the input voltage.