(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 circuits have two purposes in synthesizers. Every analog synthesizer includes at least one in its keyboard interface circuit. The reason for this is: an analog "resistor ladder" keyboard generates a control voltage that is proportional to the key that is pressed, but it will only continue to do so as long as the key is held down; when the key is released, the control voltage output returns to (usually) zero or a minimum value. However, depending on envelope generator settings, the note that was played may continue to sound for some time after the key is released. If the sample-and-hold were not in the keyboard interface circuit to hold the control voltage of the key that was played, the sounding note would jump to a very low pitch the moment that the key was let up and the control voltage returned to zero.
Sample and hold circuits are also sometimes made available for use as a general-purpose control voltage source in a patch. A common paradigm is to use an LFO as the trigger, so that the S/H samples its input on a repeating basis. If, for example, a sawtooth wave is input as the signal to be sampled, the S/H will produce a staircase output. A frequent and somewhat cliched use is to sample white noise, resulting in a sequence of totally random voltages; this is then input to a VCO to play completely random notes, or to a VCF to produce a unique "jibberish talking" filter effect.