A phenomonen which can happen to some integrated circuits when they are hit with input voltages which exceed the power supply voltages -- for instance, from a control voltage input. The circuit stops responding to its inputs, and the power must be removed to reset it. Latchup can cause circuit damage. Analog synthesizers from the 1970s and early 1980s, with external control voltage inputs, can be particularly vulnerable to this problem. So the user should be particularly careful when driving these from external systems such as modular synthesizers, which are usually capable of producing higher control voltages.