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.

