An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on thermionic emission of electrons from the electrodes supporting the arc. An archaic term is voltaic arc, as used in the phrase "voltaic arc lamp".

The phenomenon was first described in 1802, as a "special fluid with electrical properties", by Vasily V. Petrov, a Russian scientist experimenting with a copper-zinc battery consisting of 4200 discs. Sir Humphry Davy first demonstrated the arc early in the nineteenth century by transmitting an electric current through two touching carbon rods and then pulling them a short distance apart. In 1801, at a lecture before the Royal Society, he produced a "feeble" arc, not readily distinguished from a sustained spark, between charcoal points. The Society subscribed for a more powerful battery of 1000 plates and in 1808 he demonstrated the large-scale arc. He is credited with naming the arc.