A technical note on varistors by Nippon Chemi-Con Corporation. Link here (PDF)
A varistor has the volt-ampere characteristics in which current suddenly starts to flow through the device at a certain voltage. Varistors are used to protect semiconductor devices in electronic and electric circuits from overvoltage. They are inserted in parallel with a circuit to be protected. When an impulse is applied to the circuit, pulse current flows to limit the pulse voltage to the varistor limit voltage Vclamp.