As everyone in here has said, it was a mistake, but I've noticed no one has explained why it was a mistranslation, so I'm going to. In Japanese roomaji (the Arabic letter representations of Japansese hiragana and katakana) Aerith is spelt Earisu. Unfortunately, the Japanese katakana has not yet come up with a symbol to represent the "th" sound (and probably never will because the Japanese people can't pronounce it anyways), so they use the "s" sound instead. Hence, the localization team for the U.S. mistranslated Earisu as Aeris, instead of Aerith as it was supposed to be. Understandable. Man, it's a good thing they didn't make the same mistake with Sephiroth. Can you imagine that? Having people call him Sephiros? That's just so wrong on so many levels. *shudders* Anyways, that's why the name Aeris circulates, but it is incorrect.