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 22241.
Source
40 CFR § 1036.801
Scoping language
The following definitions apply to this part. The definitions apply to all subparts unless we note otherwise. All undefined terms have the meaning the Act gives to them. The definitions follow: