An explosion is a sudden release of mechanical, chemical, or nuclear energy in a violent manner, usually with the generation of high temperature and the release of gases. An explosion causes pressure waves in the local medium in which it occurs. Explosions are categorized as deflagrations if these waves are subsonic and detonations if they are supersonic (shock waves).
Most common artificial explosives are chemical explosives, usually involving a rapid and violent oxidation reaction that produces large amounts of hot gas. Gunpowder was the first explosive to be discovered and put to use. Other notable early developments in chemical explosive technology were Abel's invention of nitrocellulose (guncotton) in 1865 and Alfred Nobel's invention of dynamite (stabilized nitroglycerin). See the article on explosive material for more detail on chemical explosives. A new order of explosive, the nuclear bomb, was invented in 1945 by the United States military. In 1950, the US military developed the first fusion bomb.
Explosions are common in nature. On Earth, most natural explosions arise from volcanic processes of various sorts. Explosive volcanic eruptions occur when magma rising from below has much dissolved gas in it; the reduction of pressure as the magma rises causes the gas to bubble out of solution, resulting in a rapid increase in volume. Explosions also occur as a result of Earth impacts. On other planets, vulcanism and impacts cause explosions with various frequency.
Solar flares are an example of explosion common on the Sun, and presumably on most other stars as well. The energy source for solar flare activity comes from the tangling of magnetic field lines resulting from the rotation of the Sun's conductive plasma.
Among the largest known explosions in the universe are supernovae, which result from stars exploding, and gamma ray bursts, whose nature is still in some dispute.