[firat]

Okay, I understand that the formal definition of "idempotent" is different than what the author means. What is the correct term to use in this case?

Edit: Next paragraph says:

  This is a very useful property in many situations, as it means that an operation can
  be repeated or retried as often as necessary without causing unintended effects.
  With non-idempotent operations, the algorithm may have to keep track of whether the
  operation was already performed or not.

"A change in state" would be an unintended effect I think.

[saurik]

I think the best "term" you can use here is "side effect free". I almost wanted to say "pure" would work, but there is no real requirement that GET always return the same thing: it just needs to not change the state of the server in a way that a later GET could detect (although, honestly, you really only practically care about "find bothersome", and if there is a term for "bothersome side effect free" it would probably be from medicine and not computer science).