(Often abbreviated S/H) An electronic circuit that, when activated by a trigger signal, measures the instantaneous value (usually the voltage) of an input signal, and then generates an output signal corresponding to this instantaneous value. The output signal value is then held constant (ignoring any further changes in the input signal) until the circuit is triggered again. Sample and hold is also an essential part of an A/D converter; it holds the input voltage constant while the digitization circuits convert the voltage to a digital value.