Diesel Exhaust Fluid (DEF) is a clear, non-toxic liquid used in diesel engines equipped with Selective Catalytic Reduction (SCR) systems. It is composed of 32.5% urea and 67.5% deionized water. DEF helps reduce harmful nitrogen oxide (NOx) emissions by converting them into nitrogen and water vapor, making diesel engines more environmentally friendly. DEF is stored in a separate tank within the vehicle and is injected into the exhaust stream. It is essential for meeting EPA emissions standards and is commonly used in commercial vehicles, trucks, and some passenger cars. Regular monitoring and refilling of DEF are necessary for optimal engine performance.