A fluid that is injected into diesel exhaust after it exits the engine and before it enters a catalytic converter. It's part of a larger system called Selective Catalytic Reduction (SCR). The purpose of SCR systems is to convert harmful oxides of Nitrogen (NOX) into harmless Nitrogen and water. In other words, DEF helps turn dirty diesel exhaust into something considerably less harmful to humans.
The majority (67.5%) of DEF is deionized water, while the other 32.5% is urea, an organic compound commonly used in fertilizers. DEF is stored in its own unique on board tank and typically filled through a nozzle near the gas tank fill cap. DEF freezes at around 12 degrees Fahrenheit so most vehicles are designed to heat the DEF injection system. Most vehicles will run thousands of miles on one small tank of DEF so fill ups generally coincide with typical oil change intervals. AdBlue is a brand name for DEF used by German automotive companies.