Proximity fuze

fuze that detonates an explosive device
Proximity fuze MK53 fuze removed from shell. Circa 1950s

A proximity fuze (also called a VT fuze, for "variable time") is a fuze that is made to create an explosion when the distance to the target becomes smaller than a set value, or when the target passes through a given line. Proximity fuzes are mostly used in anti-aircraft warfare. They usually work as a simple radar.