The term Ground Zero may be used to describe the point on the earth's surface where an explosion occurs. In the case of an explosion above the ground, Ground Zero refers to the point on the ground directly below explosion (see hypocenter).
The term has often been associated with nuclear explosions and other large bombs, but is also used in relation to earthquakes, epidemics and other disasters to mark the point of the most severe damage or destruction. Damage gradually decreases with distance from this point.

History of term
Relating to a specific event, the term was first used to refer to the devastation caused by the atomic bombings of Hiroshima and Nagasaki [1].

World Trade Center
The term is often re-used for disasters that have a geographic or conceptual epicenter.