[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.
[glossary]