DEF
Diesel exhaust fluid (DEF) means a liquid reducing agent (other than the engine fuel) used in conjunction with selective catalytic reduction to reduce NOX emissions. Diesel exhaust fluid is generally understood to be an aqueous solution of urea conforming to the specifications of ISO 18611 or ISO 22241.
Source
40 CFR § 1065.1001
Scoping language
The definitions in this section apply to this part. The definitions apply to all subparts unless we note otherwise. All undefined terms have the meaning the Act gives them. The definitions follow: