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A word or more words used by a searcher on a search engine. In SEO, keywords are the words that a website is optimized to rank for and in PPC, keywords are bid on by advertisers. In Online Reputation Management, a keyword is a term that is used when searching the Internet for mentions.

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 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 ALGOL and PL/I there are keywords but no reserved words, with keywords being distinguished from identifiers by other means.

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