Liquefied petroleum gases (LPG): A group of hydrocarbon gases, primarily propane, normal butane, and isobutane, derived from crude oil refining or natural gas processing. These gases may be marketed individually or mixed. Liquefied petroleum gas, also referred to as LPG or LP gas, is� a fuel gas which contains a flammable mixture of hydrocarbon gases, specifically propane, n-butane and isobutane. LPG, any of several liquid mixtures of the volatile hydrocarbonâ??s propene, propane, butene, and butane. It was used as early as 1860 for a portable fuel source, and its production and consumption for both domestic and industrial use have expanded ever since. A typical commercial mixture may also contain ethane and ethylene, as well as a volatile mercaptan, an odorant added as a safety precaution.