Source : WordNet®

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

Source : Free On-Line Dictionary of Computing

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)