Any material capable of a sudden release of energy and the rapid formation of a large volume of gas, leading when compressed to the development of a high-pressure wave (blast).

Combustion and explosion differ essentially only in rate of reaction, and many explosives (called low explosives) are capable of undergoing relatively slow combustion under suitable conditions. High explosives produce uncontrollable blasts.

The first low explosive was gunpowder ; the first high explosive was nitroglycerine. In 1867, Alfred Nobel produced dynamite by mixing nitroglycerine with kieselguhr, a fine chalk like material.

Other explosives now in use include trinitrotoluene (TNT); ANFO (a mixture of ammonium nitride and fuel oil), which is widely used in blasting; and pentaerythritol tetranitrate (PETN), a sensitive explosive with high power.

Military explosives are often based on cyclonite (also called RDX), which is moderately sensitive but extremely powerful. Even more powerful explosives are made by mixing RDX with TNT and aluminium.

Plastic explosives, such as Semtex, are based on RDX mixed with oils and waxes. The explosive force of atomic and hydrogen bombs arises from the conversion of matter to energy according to Einstein's mass-energy equation, E = mc^2^.