Reserved word
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ja:予約語 A reserved word is a word which, in some computer programming language, cannot be used as an identifier because it is already used for some grammatical purpose. For instance, in SQL, a user cannot be called 'group' because the word 'group' is used to indicate that an identifier refers to a group, not a user. Such a word is a keyword; it is because its use is restricted that it is also a reserved word.
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 without "breaking" older Java source code.