Reserved words are words that are part of the standard C language library.

A reserved word, also referred to as a reserved identifier, is a term that is “reserved from use” in computer languages and cannot be used as an identifier, such as the name of a variable, function, or label.

In a computer language, a reserved word (also known as a reserved identifier) is a word that cannot be used as an identifier, such as the name of a variable, function, or label – it is “reserved from use”. This is a syntactic definition, and a reserved word may have no user-defined meaning.

A closely related and often conflated notion is a keyword, which is a word with special meaning in a particular context. This is a semantic definition. By contrast, names in a standard library  but not built into the language are not considered reserved words or keywords. The terms “reserved word” and “keyword” are often used interchangeably – one may say that a reserved word is “reserved for use as a keyword” – and formal use varies from language to language; for this article we distinguish as above.

In general reserved words and keywords need not coincide, but in most modern languages keywords are a subset of reserved words, as this makes parsing easier, since keywords cannot be confused with identifiers. In some languages, like C or Python, reserved words and keywords coincide, while in other languages, like Java, all keywords are reserved words, but some reserved words are not keywords – these are “reserved for future use”. In yet other languages, such as the older languages ALGOL, FORTRAN and PL/I, there are keywords but no reserved words, with keywords being distinguished from identifiers by other means.

Reserved words, also known as keywords, are words in a programming language that have special meanings and are reserved for specific purposes. These words cannot be used as identifiers (such as variable names or function names) because they are already predefined and used by the language itself. The set of reserved words varies depending on the programming language. Here are some examples of reserved words in different programming languages:

1. C/C++:
   • if, else, while, do, for: Control flow statements
   • int, char, float, double: Data types
   • return: Used to exit a function and return a value
2. Java:
   • if, else, while, do, for: Control flow statements
   • int, char, float, double: Data types
   • return: Used to exit a method and return a value
   • class, public, private: Keywords related to class and access modifiers
3. Python:
   • if, else, while, for: Control flow statements
   • def, class: Used for defining functions and classes
   • True, False, None: Boolean and null-like values
4. JavaScript:
   • if, else, while, for: Control flow statements
   • var, let, const: Used to declare variables
   • function, class: Keywords for defining functions and classes
5. Ruby:
   • if, else, while, do, for: Control flow statements
   • def, class: Used for defining methods and classes
   • true, false, nil: Boolean and null-like values
6. PHP:
   • if, else, while, do, for: Control flow statements
   • function, class: Keywords for defining functions and classes
   • true, false, null: Boolean and null-like values

These are just a few examples, and each programming language has its own set of reserved words. It’s important to become familiar with the reserved words of the language you are working with to avoid conflicts and errors in your code.