Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents (including water, coal-gas and coal-tar) are driven off by baking in an airless oven at temperatures as high as 1,000 degrees Celsius so that the fixed carbon and residual ash are fused together. The carbon content of coke is partially converted to graphite.

Coke typically has a specific gravity in the range 1.85 - 1.9. It is highly porous, and a mass of coke has 40% greater volume than the equivalent mass of coal.

Since the smoke-producing constituents are driven off during the coking of the coal, coke forms a desirable fuel for stoves and furnaces in which conditions are not suitable for the complete burning of bituminous coal itself. Coke may be burned with little or no smoke under combustion conditions which would result in a large amount of smoke if bituminous coal were the fuel. Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Coke from coal is grey, hard, and porous and has a heating value of 28 megajoules/kilogram.

The use of coke as a fuel was pioneered in 17th century England in response to the ever-growing problem of European deforestation. Wood was becoming increasingly scarce and expensive, and coal's fumes, particularly smoke and sulfur compounds, disqualified it from many applications, including cooking and iron smelting. In 1603, Sir Henry Platt suggested that coal might be charred in a manner analogous to the way charcoal is produced from wood. This process was not put into practice, however, until 1642, when coke was used for roasting malt in Derbyshire. (Coal could not be used in brewing, because its sulfurous fumes would impart a foul taste to the resulting beer.) Perhaps more significantly, in 1709, Abraham Darby set up a coke-fired blast furnace to produce cast iron. The ensuing availability of inexpensive iron was one of the factors leading to the European industrial revolution.

The solid residue remaining from the refinement of petroleum by the "cracking" process is also a form of coke. Petroleum coke has many uses besides being a fuel, such as the manufacture of dry cells, electrodes, etc. Gas works that manufacture syngas also produce coke as an end product, called gas house coke.