idempotent: [from mathematical techspeak] adj. Acting as if used only once,

:idempotent: [from mathematical techspeak] adj. 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 AI Hackers Dictionary