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.
In many languages, such as C and similar environments like C++, 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.
In some languages, such as ALGOL and Algol 68, keywords cannot be written verbatim, but must be stropped. This means that keywords must be marked somehow. E.g. by quoting them or by prefixing them by a special character.
Some languages, such as PostScript, are extremely liberal in this approach, allowing core keywords to be redefined for specific purposes.
In Common Lisp, the term “keyword” (or “keyword symbol”) is used for a special sort of symbol, or…