Electrical device inserted in a power line to protect equipment from sudden fluctuations in current WordNet 3.0 & Open English WordNet

A lightning arrester is a device, essentially an air gap between an electric wire and ground, used on electric power transmission and telecommunication systems to protect the insulation and conductors of the system from the damaging effects of lightning. Wikipedia

A surge protector Wikipedia

Device in power/telecommunication systems that protects against lightning, such that when a lightning surge happens, the current is diverted through the arrester to earth Wikidata

Electrical appliance Wikidata

A device used on electrical and telecommunications systems to protect the insulation and conductors from the damaging effects of lightning. Typically, it has a high-voltage terminal and a ground terminal. When a surge travels along the power line to the arrester, the current from the surge is diverted through the arrester, in most cases to earth. Wiktionary

An appliance designed to protect electrical devices from voltage spikes by limiting the voltage supplied to an electric device by either blocking or by shorting to ground any unwanted voltages above a safe threshold. Wiktionary

Appliance designed to protect electrical devices. Wiktionary (translation)

A surge protector. Wiktionary