Wikipedia: Ground zero
The term ground zero (sometimes also known as surface zero as distinguished from zero point) 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 an 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. The term is often re-used for disasters that have a geographic or conceptual epicenter.
History of term
The origins of the term ground zero began with the Manhattan Project and the bombing of Japan. The Oxford English Dictionary, citing the use of the term in a 1946 New York Times report on the destroyed city of Hiroshima, defines "ground zero" as "that part of the ground situated immediately under an exploding bomb, especially an atomic one."
The term was military slang — used at the Trinity site where the weapon tower for the first nuclear weapon was at "point zero" — and moved into general use very shortly after the end of World War II.
World Trade Center
In and around New York City, "Ground Zero" is generally understood to mean the site of the World Trade Center, which was destroyed in the September 11, 2001 attacks. The phrase was being applied to the World Trade Center site within hours after the towers collapsed. The adoption of this term by the mainstream North American media with reference to the September 11th attacks began as early as 7:47 a.m. (EDT) on that day, when CBS News reporter Jim Axelrod said,
“Less than four miles behind me is where the Twin Towers stood this morning. But not tonight. Ground Zero, as it's being described, in today's terrorist attacks that have sent aftershocks rippling across the country."
Wiktionary: ground zero
ground zero (plural ground zeroes)
1. The exact location on the ground marking the detonation point of any bomb
2. (by extension) The location of any disaster
(Oxford English Dictionary)
ground zero, that part of the ground situated immediately under an exploding bomb, esp. an atomic one.
1946 N.Y. Times 7 July E10/1 The intense heat of the blast started fires as far as 3,500 feet from ‘*ground zero’.
1955 Bull. Atomic Sci. Sept. 255/1 There was no noticeable contamination even at ground zero at Hiroshima.
30 June 1946, New York (NY) Times, pg. 3:
The most instructive fact at Nagasaki was the survival, even when near ground zero (the spot below which the bomb exploded) of the few hundred people who were properly placed in the tunnel shelters, though unoccupied, stood up well in both cities.