Comment by 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.