Reserved Identifier

In a programming language, a reserved word (also known as a reserved identifier or keyword) is a word with special meaning, which generally cannot be used as usual identifiers, such as variable names or function names. For example, in most procedural language, the word "if" is reserved, as it indicates a conditional, and cannot be used as a variable name. These words are predefined in the language’s formal specifications. Typically, reserved words include labels for primitive data types in languages that support a type system, and identify programming constructs such as loops, blocks, conditionals, and branches.

The list of reserved words in a language are defined when a language is developed. Occasionally, depending on the flexibility of the language specification, vendors implementing a compiler may extend the specification by including non-standard features. Also, as a language matures, standards bodies governing a language may choose to extend the language to include additional features such as object-oriented capabilities in a traditionally procedural language. Sometimes the specification for a programming language will have reserved words that are intended for possible use in future versions. In Java, const and goto are reserved words — they have no meaning in Java but they also cannot be used as identifiers. By "reserving" the terms, they can be implemented in future versions of Java, if desired, without "breaking" older Java source code. Reserved words may not be redefined by the programmer, unlike predefined functions, methods, or subroutines, which can often be overridden in some capacity. The name of a predefined function, method, or subroutine is typically categorized as an identifier instead of a reserved word.