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❪֎₆ₐ❫ Keywords ○｜Definition｜1st｜20260506164056-00-⌔
Reserved word
In a programming language, a reserved word (sometimes known as a reserved identifier) is a word that cannot be used by a programmer as an identifier, such as the name of a variable, function, or label – it is “reserved from use”. In brief, an identifier starts with a letter, which is followed by any sequence of letters and digits (in some languages, the underscore ’_’ is treated as a letter).
In an imperative programming language and in many object-oriented programming languages, apart from assignments and subroutine calls, keywords are often used to identify a particular statement, e.g. if, while, do, for, etc. Many languages treat keywords as reserved words, including Ada, C, C++, COBOL, Java, and Pascal. The number of reserved words varies widely from one language to another: C has about 30 while COBOL has about 400.
A few languages do not have any reserved words; Fortran and PL/I identify keywords by context, while Algol 60 and Algol 68 generally use stropping to distinguish keywords from programmer-defined identifiers, e.g.
.ifor'ifor'if'orifis a keyword distinct from identifierif.Most programming languages have a standard library (or libraries), e.g. mathematical functions sin, cos, etc. The names provided by a library are not reserved, and can be redefined by a programmer if the library functionality is not required.
Printed 2026-06-28.
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