"The explosion of a nuclear (or atomic) weapon at the surface of the land or water at a height above the surface less than the radius of the fireball at maximum luminosity (in the second thermal pulse). An explosion in which the weapon is detonated actually on the surface is called a contact surface burst or a true surface burst". (Glasstone, The Effects of Nuclear Weapons, 3rd Ed., 1977, Glossary, p. 640)