Fuel refers to any substance that can react with other substances and release energy. Used as heat or for work. The concept was initially applied only to materials capable of releasing chemical energy, but has since been applied to other sources of thermal energy, such as nuclear energy (through fission and fusion).

The heat energy released by fuel reaction is converted into mechanical energy by heat engine. At other times, the heat itself is used for warmth, cooking or industrial processes, as well as lighting from combustion. Fuel can also be used in biological cells through a process called cell respiration, in which organic molecules are oxidized to release available energy. So far, hydrocarbons and related oxygen-containing molecules are the most commonly used fuel sources for human beings, but other substances, including radioactive metals, are also used.

Fuel is in contrast to other substances or devices that store potential energy, such as direct release of electrical energy (such as batteries and capacitors) or mechanical energy (such as flywheel, spring, compressed air or water in water reservoirs).

Gasoline (American English) or gasoline (British English) is a transparent liquid derived from petroleum, which is mainly used as the fuel of spark ignition internal combustion engine. It is mainly composed of organic compounds obtained by fractionating petroleum, and various additives are added. When processed in refineries, an average of 42 gallons (159 liters) per barrel of crude oil can produce about 19 US gallons (72 liters) of gasoline, although this varies depending on the method of analysis of the source of the crude oil.

The performance of a specific gasoline mixture against premature ignition, which can cause detonation and reduce the efficiency of reciprocating engines, is measured by the octane number. Gasoline is produced in a variety of octane rating. Tetraethyl lead and other lead compounds are no longer used in most fields to regulate and improve octane number, but many other additives are introduced into gasoline to improve its chemical stability, control corrosiveness, provide fuel system cleaning and determine performance characteristics for the intended use. For economic, political or environmental reasons, sometimes gasoline also contains ethanol as an alternative fuel.