the explosion of a nuclear weapon at such a height that the expanding fireball does not contact the Earth's surface

a nuclear weapon explosion that is high enough in the air to keep the fireball from touching the ground. Because the fireball does not reach the ground and does not pick up any surface material, the radioactivity in the fallout from an air burst is relatively insignificant compared with a surface burst. For more information, see Chapter 2 of CDC's Fallout Report at http://www.cdc.gov/nceh/radiation/fallout/falloutreport.pdf.

An explosive e.g. artillery round, rigged to detonate above ground / enemy troops

nuclear experiment above the ground.

An air burst occurs whenever an explosive device such as an anti-personnel artillery shell or a nuclear weapon is detonated in the air instead of on contact with the ground or target or a delayed armor piercing explosion.