What is DEF? Here's the technical jargon: It is an aqueous urea solution (AUS 32) made with 32.5% urea and 67.5% deionized water used in selective catalytic reduction (SCR) to reduce nitrogen oxide (NOx) concentration in diesel exhaust. So, what does that mean in English? Let's start from the beginning. The United States Environmental Protection Agency (EPA) has been around...