Is it Aeris or Aerith?

In Final Fantasy VII, I've always known the one of the lead protagonists to be named Aeris, and this has always fit with me because an Aeris is a name for a person who sells flowers. Almost everyone I've ever known though who completely obsess over the FFVII universe always try and rudely correct me and tell me that it's Aerith not Aeris. Which is it and why the confusion?