An oil which 1) is liquid at room temperature (approximately 20 degrees Celsius) 2) is intended to be burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, 3) is obtained from petroleum distillation, either as a distillate or a residue, 4) composed of long hydrocarbon chains, particularly alkanes, cycloalkanes and aromatics.

Oils having a flash point of approximately 42 degrees Celsius and oils burned in cotton or wool-wick burners are not typically considered fuel oils. The term fuel oil is also used in a stricter sense to refer only to the heaviest commercial fuel that can be obtained from crude oil, i.e., heavier than gasoline and naphtha.