Explosive, a substance that will burn up or decompose almost instantly, producing intense heat and a large volume of expanding gas. This rapid change of state is called an explosion. Depending on its nature, an explosive may be detonated (set off, or exploded) by applying heat (such as a flame or spark), by shaking, or by striking. The sharp sound, or report, of an explosion is due to air waves set in motion by the explosion's force. This article discusses chemical explosives. Fuels such as gasoline and diesel oil are not considered explosives, even though they can be caused to explode.

Explosives have both constructive uses, as in building roads, railways, and tunnels, and destructive uses, as in war. Ammunition and blasting make use of the expanding gases that result from an explosion. Fireworks use the light and noise.

Explosives may be chemical substances mixed together by mechanical means (gunpowder is an example) or they may be chemical compounds (such as TNT). The explosion is caused by a chemical reaction set off by shock or heat. In explosives such as gunpowder (a low explosive) this reaction takes the form of rapid combustion (burning). In an explosive such as TNT (a high explosive) the reaction is a rapid decomposition, or breaking down, of the compound into its parts.

Low explosives can be set off by impact, heat, a spark, or an electric charge; high explosives generally require an explosion to set them off. When ignited, a low explosive is converted into gas at a relatively slow rate, a high explosive at a much more rapid rate. An explosion will move through a low explosive at a speed less than that of sound. In a high explosive, an explosion will move at a speed up to several times that of sound, creating a shock wave that produces a shattering effect. Some explosives, in particular low explosives, generally do not explode unless they are confined within a casing or within a hole.