Comment by jholman

I don't disagree with common usage, but common usage is not what's being discussed here.

It is a side effect in programming usage (not just HTTP).

Something is side-effect-free if and only if the only result of it running is that you get an answer. If you ignore the answer, then you cannot tell you ran the function/method/call/whatever. PUT is not side-effect-free.

That said, side-effect-free-ness is an incomplete paraphrasing of the HTTP spec (RFC7231); you'll notice that the only mentions of the phrase "side effect" are giving examples of legal side effects.