An airburst is a
detonation that takes place above the
surface of the earth, as opposed to the ground-level
ground burst. This is usually meant in the context of the
effects of nuclear weapons, but can also refer to
high explosive munitions and
fuel-air explosives. The primary reason for detonating a weapon above the ground is to maximize the area exposed to some level of the explosion's effects.
For example, when shooting a nuclear warhead at an opposing missile silo, a targeter would likely use a ground burst in order to concentrate maximum damage on the silo itself. As a tradeoff, the area affected by the blast will be small due to the earth's curvature and the tendency of the ground to absorb the blast. However, if shooting at a city, the weapon can be detonated much higher, since a lower level of effects are required to destroy softer targets such as civilian infrastructure. In this case, an airburst would be used. If the targeter knows what level of damage or what type of effects he or she wishes to subject the maximum area to, as well as the power of the explosion, it is fairly easy to derive the proper altitude for maximum damage expectancy.
Airbursts actually produce the least fallout, since there is not as much material sucked up into the fireball and irradiated. The Hiroshima and Nagasaki nuclear strikes were both airbursts.