I came across this word when reading about pipelining at mozilla.org. They used it casually, as if everyone knows what it means. I looked it up and it’s actually a useful word, at least in the context of computer science.

In computing, idempotence is the quality of something that has the same effect if used multiple times as it does if used only once…

There are a lot of times where knowing whether or not a function is idempotent would be handy, especially with stuff like character-escaping.