Propane is a three-carbon alkane with the molecular formula C₃H₈, normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central heating. Propane is one of a group of liquefied petroleum gases. The others include butane, propylene, butadiene, butylene, isobutylene and mixtures thereof.

Propane as HD-5 was originally intended for use as vehicle fuel. HD-5 is currently being used in all propane applications. Propane containing too much propylene is not suited for most vehicle fuels. HD-5 is a specification which establishes a maximum concentration of 5% propene in propane. Propane and other LP gas specifications are established in ASTM D-1835. All propane fuels include an odorant, almost always ethanethiol, so that people can easily smell the gas in case of a leak.

Propane was first identified as a volatile component in gasoline by Walter O. Snelling of the U.S. Bureau of Mines in 1910. The volatility of these lighter hydrocarbons caused them to be known as "wild" because of the high vapor pressures of unrefined gasoline. On March 31, the New York Times reported on Snelling's work with liquefied gas and that "...a steel bottle will carry enough [gas] to light an ordinary home for three weeks."