An electric discharge due to the discharge of one of the charged regions of a thunder cloud. The polarity of the lightning stroke is the polarity of the electric charge that comes to earth. A complete lightning flash is a complete discharge along a single path.

The path of the lightning flash is established by an initial discharge, the leader stroke, that can develop either downwards from the cloud to earth or upwards from earth to the cloud. A dart leader stroke is a leader stroke that develops continuously. One that develops in a series of relatively short steps is a stepped leader stroke. The return stroke consists of a high current discharge that flows upwards as soon as a downward leader stroke strikes the earth. If the flash is made up of more than one lightning stroke it is termed a multiple-stroke lightning flash.

A lightning stroke to any part of a power or communication system is described as a direct stroke and the surge produced in the system by it or by a flashover from it is a direct lightning surge. An indirect stroke induces a voltage in such a system without actually striking it and the surge induced by it is an indirect lightning surge.

A lightning protective system is a complete system of conductors that is designed to protect a building or equipment from the effects of a lightning stroke. See also lightning conductor.