What fluid is injected into diesel exhaust to reduce nitrogen oxide emissions?

The fluid injected into diesel exhaust to reduce nitrogen oxide emissions is diesel exhaust fluid.

Diesel exhaust fluid, or DEF, is injected into the hot exhaust stream in vehicles equipped with selective catalytic reduction. Heat causes the fluid’s urea to produce ammonia, which reacts inside the SCR catalyst with nitrogen oxides. The main products are nitrogen and water, substantially reducing NOx emissions.

DEF is not a fuel additive and is not injected into the engine’s cylinders. It is stored in a separate tank and consumed by the exhaust after-treatment system. Its standardized composition is 32.5% urea and 67.5% deionized water; the international designation is AUS 32.

Several options in this question are names for the same substance or closely related descriptions. DEF is the common abbreviation, AdBlue is a registered trade name widely used in Europe, and “urea solution” describes its composition. The fluid must meet quality standards because contamination can damage the SCR system.