Any instance of a character being written by anyone other than their original creator is merely fan-fiction. The big two having made a legal business practise out of it doesn't change that fact.
I disagree. It's possible to write something you're *not* a fan of, just for the money. All those interviews where the writers say, "I've always wanted to write this character, I'm so passionate about it yadda yadda", if they're not lying, then it's fanfic. If you're not a fan of the characters you're writing, then it's just plain paid fiction work, not paid fanfiction work.