Def

Def is a term with multiple meanings across fields, often appearing as an abbreviation or shorthand. Its sense depends on context, ranging from language and computing to engineering and sports.

In language and writing, def is commonly used as an abbreviation for definition. Dictionaries and glossaries may label an entry with def. or define a term by introducing a formal definition. In informal use, people sometimes write “def” to mean “definition” or “definitely,” though the latter is slang and not standard in technical writing.

In computing, def is a keyword in several programming languages. Most notably in Python, def introduces a function definition, for example: def my_function(x): return x + 1. It signals the start of a function’s name, parameters, and body. Other languages use similar constructs under different syntax.

In automotive and environmental contexts, DEF stands for Diesel Exhaust Fluid. It is a urea–water solution (about 32.5% urea) used in selective catalytic reduction systems to reduce nitrogen oxide (NOx) emissions from diesel engines. DEF is not a fuel additive but a consumable stored separately in a dedicated tank and dispensed into the exhaust stream. Proper handling, temperature management, and ISO 22241 standards govern its use.

In sports and defense contexts, DEF is commonly an abbreviation for defense or defenders on rosters and statistics. It also appears in military terminology as part of longer terms like DEFCON (defense condition), though DEFCON itself is not simply “DEF.”

Overall, Def functions as a versatile shorthand whose meaning is driven by the surrounding domain.