A varistor (also known as a voltage-dependent resistor, or VDR) is a resistor whose resistance changes as the voltage across it increases or decreases. Varistors are typically constructed from a ceramic material containing semiconductor materials such as silicon and germanium. They have a relatively large resistance at low voltage levels, but as the voltage increases, their resistance drops significantly, allowing more current to flow through them. This feature makes them useful for absorbing transient voltages, which are sudden increases in the voltage that can occur when a device is powered on or off. Varistors are also commonly used to provide overvoltage protection, where they absorb and dissipate excessive voltages before they reach sensitive electronic components.