What is DEF? If you are not using DEF now chances are you will be soon. DEF stands for Diesel Exhaust Fluid, which is an operating fluid for new engine technology. This fluid is non-hazardous, non-toxic, non-flammable, non-explosive that is very safe to use. It is used to chemically reduce NOx emissions from vehicles powered by diesel engines. Campbell Oil Company offers a wide variety of different option for Diesel Exhaust Fluid (DEF). Contact us today for different tank packages and installation quotes.