Aerith

Aerith is an alternative romanisation of Aeris. It is believed that the spelling used in Final Fantasy VII was a mistranslation, and most English-language sources featuring her since then have spelt her name as Aerith- the most obvious so far being Kingdom Hearts.

Since she is also credited as Aerith in the English dub of Final Fantasy VII: Advent Children (nobody ever says her name in the film itself, so we can't check pronunciation) it appears that her name has been retconned even within the world of FFVII itself. Guess we'll have to wait and see if that remake ever happens to know for sure.