Diesel Exhaust Fluid (DEF) is a non-toxic solution made of 32.5% urea and 67.5% deionized water. It is used in selective catalytic reduction (SCR) systems to reduce harmful nitrogen oxide (NOx) emissions from diesel engines. DEF is injected into the exhaust stream, where it reacts with NOx to convert it into harmless nitrogen and water vapor. DEF is essential for meeting environmental regulations and improving air quality. It is stored in a separate tank in vehicles and must be replenished regularly. Proper handling and storage are important, as DEF can degrade if exposed to high temperatures or contaminants.