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The Free On-line Dictionary of Computing (foldoc)

1. A function f : D -> D is idempotent if

f (f x) = f x for all x in D.

I.e. repeated applications have the same effect as one. This
can be extended to functions of more than one argument,
e.g. Boolean & has x & x = x. Any value in the {image} of an
idempotent function is a {fixed point} of the function.

2. This term can be used to describe {C} header files, which
contain common definitions and declarations to be included by
several source files. If a header file is ever included twice
during the same compilation (perhaps due to nested #include
files), compilation errors can result unless the header file
has protected itself against multiple inclusion; a header file
so protected is said to be idempotent.

3. The term can also be used to describe an initialisation
subroutine that is arranged to perform some critical action
exactly once, even if the routine is called several times.

[{Jargon File}]

(1995-01-11)