An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An explosive charge is a measured quantity of explosive material. The material may either be composed solely of one ingredient or be a mixture containing at least two substances.

The potential energy stored in an explosive material may, for example, be:

Explosive materials may be categorized by the speed at which they expand. Materials that detonate, in which the front of the chemical reaction moves faster through the material than the speed of sound, are called “high explosives.” In contrast, materials that deflagrate, where the front of the reaction moves more slowly than the speed of sound, are known as “low explosives.” Explosives may also be categorized by their sensitivity. Sensitive materials that can be initiated by a relatively small amount of heat or pressure are primary explosives, and materials that are relatively insensitive are secondary or tertiary explosives.

A wide variety of chemicals can explode; only some are manufactured specifically for the purpose of being used as explosives. The remainders are too dangerous, sensitive, toxic, expensive, unstable, or prone to decomposition or degradation over short time spans.

In contrast, some materials are merely combustible or flammable if they burn without exploding. The distinction, however, is not always clear. Certain materials—dusts, powders, gases, or volatile organic liquids—may be simply combustible or flammable under ordinary conditions, but become explosive in specific situations or forms, such as dispersed airborne clouds, or confinement or sudden release.

Early thermal weapons, such as Greek fire, have existed since ancient times. The history of chemical explosives is closely intertwined with the history of gunpowder. While searching for an elixir of eternal life, Taoist alchemists in China created the earliest form of gunpowder from coal, saltpeter, and sulfur. Gunpowder was the first form of chemical explosives, first seeing use in warfare in 1161. Early forms of explosives in warfare included bamboo firecrackers, which were explosives fired from bamboo or bronze tubes.

The first explosive stronger than black powder to see widespread use was nitroglycerin, developed in 1847. Since nitroglycerin is a liquid and highly unstable, it was replaced by nitrocellulose and trinitrotoluene (TNT) in 1863, smokeless powder and dynamite in 1867, and gelignite. World War I saw the adoption of TNT in artillery shells, while World War II saw extensive use of new explosives .

In modern weapons, these have largely been replaced by more powerful explosives such as C-4 and pentaerythritol tetranitrate (PETN), which are waterproof and malleable, though they may catch fire due to reactions with metals.