An iteration is a repetition of something. Your nagging friend's message might be lost on you after the umpteenth iteration.
The noun iteration comes from the Latin verb iterum, which means "again," which makes a lot of sense when you consider the meaning of the word, "the act of repeating." Don’t get confused by the similar word reiteration, which essentially means the same thing. Either word is proper to use. Although when you think about it, the "re" means again anyway, so that word actually means "to repeat again," which is in itself a little redundant, don't you think?