WordNet (r) 3.0 (2006):

idempotent
    adj 1: unchanged in value following multiplication by itself;
           "this matrix is idempotent"

The Jargon File (version 4.4.7, 29 Dec 2003):

idempotent
 adj.

    [from mathematical techspeak] Acting as if used only once, even if used
    multiple times. This term is often used with respect to 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. The term
    can also be used to describe an initialization subroutine that is arranged
    to perform some critical action exactly once, even if the routine is called
    several times.


The Free On-line Dictionary of Computing (30 December 2018):

idempotent

   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)