Gas oil is a liquid fuel refined from crude oil. It is a middle distillate that has many different names in the UK and a variety of uses. Gas oil is a "dual purpose" fuel that has a high calorific value and is recommended as a boiler fuel in industrial or agricultural installations with pressure jet burners, such as grain dryers. It can also be used in a diesel engine fuelling off-the-highway equipment such as farm tractors, construction equipment, railway and marine engines, and stationary diesel engines or generators. Gas oil is also known as red diesel, farm diesel, ULSG, Class A2 or Class D, 35 second burning oil, and marked EN590.

Gas oil is a broad term that can refer to a range of intermediates and finished petroleum products, generally in the diesel or VGO range of distillation profile. It is a hydrocarbon oil used as a fuel oil, especially a petroleum distillate intermediate in boiling range and viscosity between kerosene and lubricating oil.

In summary, gas oil is a liquid fuel refined from crude oil that has a variety of uses, including as a boiler fuel and in off-the-highway equipment such as farm tractors and construction equipment. It is also known as red diesel, farm diesel, and ULSG.