Interestingly, the mathematical definition (and functional programming definition) of idempotency is slightly different. Or I've seen it used slightly different

Interestingly, the mathematical definition (and functional programming definition) of idempotency is slightly different. Or I've seen it used slightly differently, anyways - specifically, a function is idempotent if the result of f(x) is the same result as f(f(x)). I've seen this lead to some confusing back and forth between software engineers, where one engineer means one definition, and the other engineer meant the other definition.