Idempotent:
Sorry about the verbiage about their APIs, that’s a bit extraneous.
In some ways, it sounds like the formal definition of a function in mathematics: given the same inputs, the same output should always occur. For you software types, there’s no such thing as a global variable in formal mathematics.
In any case, this word was noted in “(A few) Ops Lessons We All Learn The Hard Way,” Signs of Triviality:
16. Very few operations are truly idempotent.
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