Technically Square didn't re-use Terra. In the original Japanese version, she was named Tina.If you want your mind blown, it applies even to FF 10's rikku.
Nintendo of America decided to give her a more "exotic/mysterious" name, since they didn't give a shit about accurate translations back then.
Edit: Dalthine beat me to it, but I'm not deleting this post since I stand by my last statement.