An Air Burst is when an explosive device such as an anti-personel artillery shell or a nuclear weapon is detonated in the air instead of on contact with the ground or target or a delayed armor peircing explosion.
History
Air bursts were used in the first World War to shower enemy positions and men with shrapnel to kill the largest possible number of them with a single burst, assuming that the blast was directly over the trench the men were positioned in.
Tactic
With nuclear weapons the air burst, usually several hundred feet in the air, allows the shockwave of the fission or fusion driven explosion to destroy the largest possible number of buildings, military units or vehicles, etc.
Recent Years
Newer anti-personel land mines such as the "Bouncing Betty " fire a grenade into the air which detonates somewhere between five and half and six feet, causing the shrapnel to fly out at head or chest level, decimating any military personel in a fifteen feet radius.