In computer programming, a keyword is a word or identifier that has a particular meaning to the programming language. The meaning of keywords -- and, indeed, the meaning of the notion of keyword -- differs widely from language to language. “Programming” redirects here. ... For other uses, see Word (disambiguation). ... Identifiers (IDs) are lexical tokens that name entities. ... A programming language is an artificial language that can be used to control the behavior of a machine, particularly a computer. ...

In many languages, such as C and its kin, a keyword is a reserved word which identifies a syntactic form. Words used in control flow constructs, such as if, then, and else are keywords. In these languages, keywords cannot also be used as the names of variables or functions. C is a general-purpose, block structured, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system. ... In computer programming languages, a reserved word is a word which has a special grammatical meaning to a language and cannot be used as an identifier in that language. ... In computer science control flow (or alternatively, flow of control) refers to the order in which the individual statements, instructions or function calls of an imperative or functional program are executed or evaluated. ...

Some languages, such as PostScript, are extremely liberal in this approach, allowing core keywords to be redefined for specific purposes. For the literary term, see Postscript. ...

In Common Lisp, in contrast, keywords (or keyword symbols) are a special sort of symbol, or identifier. Unlike other symbols, which usually stand for variables or functions, keywords are self-quoting and evaluate to themselves. Keywords are usually used to label named arguments to functions, and to represent symbolic values. Common Lisp, commonly abbreviated CL, is a dialect of the Lisp programming language, published in ANSI standard X3. ... Quoting, or to quote, is to encapsulate information within a pair of double quotes (ASCII code 0x22). ...

Languages vary as to what is provided as a keyword and what is a library routine. Some languages, for instance, provide keywords for input/output operations whereas in others these are library routines. In Python and many BASIC dialects, print is a keyword. In contrast, the C and Lisp equivalents printf and format are functions in the standard library. In computer science, a library is a collection of subprograms used to develop software. ... Python is a high-level programming language first released by Guido van Rossum in 1991. ... This article is about the programming language. ...

In languages with macros or lazy evaluation, control flow constructs such as if can be implemented as macros or functions. In languages without these expressive features, they are generally keywords.