Thermite is a mixture of powdered aluminum and iron oxide (Fe₃O₄) in equivalent amounts, used in welding and incendiary bombs. On ignition with a barium peroxide or magnesium fuze, a violently exothermic oxidation reaction occurs, producing molten iron at 2,500°C and alumina slag. It thus supplies both the heat and the metal for welding, and can be used to join large parts in a preheated refractory mold.

Thermite is the basis of the thermite process, a smelting process in which a metallic oxide is reduced to the metal by heating with finely divided aluminum powder. It is the basis of the Goldschmidt process for extracting such high melting-point metals such as chromium, manganese, molybdenum, and vanadium.