idempotent

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.