A visible disruptive discharge of electricity between two points of high potential difference, preceded by ionization of the path. A sharp crackling noise occurs because of the rapid heating of the air through which the spark passes. The distance travelled is determined by the shape of the electrodes and the potential difference between them, and is not necessarily the shortest possible path.

Under specified conditions the distance between the electrodes is termed the spark gap. Specially designed electrodes are used to produce a spark over a given spark gap under particular conditions, as for ignition purposes in an internal-combustion engine. The insulation is self-restoring when the potential across the spark gap falls below that required to produce the spark. A spark is of much shorter duration than an arc.