From Glossary of Meteorology
A short-duration, high-amperage, electric current impulse that may sweep through an electrical network, as a power transmission network, when some portion of it is strongly influenced by the electrical activity of a thunderstorm.
This activity may take the form of a direct lightning strike, or it may be simply the release of previously induced charge on the line when a thundercloud overhead suddenly discharges itself. Such current surges flow rapidly through the lines until they find a path to the ground through arc-over at a weak insulator or by entering terminal equipment at the end of the line. Use of grounded guard wires above the power lines of a transmission system reduces the frequency of surge current difficulties, and installation of lightning arresters at sensitive terminal equipment protects the system against damage there.