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.
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