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Disney Princesses are considered female characters who are princesses by birth title, or who married into royalty to become a princess.
They officially are:
Cinderella, married Prince Charming
Sleeping Beauty(Aurora), born a princess
Snow White, born a princess
Ariel, married Prince Eric
Jasmine, born a princess
Belle, married a prince - remember the beast was originally a prince and was turned into the beast. Then he turned back into the prince once Belle fell in love with him.
and soon to be added: Tiana, of the Frog in the Princess, currently being produced by Disney.
Now, other characters have been deemed Disney Princess's in the Disney on Ice productions, and in the Disney retail shops. These characters may not have the official title, but they fit what Disney calls "The Princess Mythology". These characters were Mulan, Pocohantas, and Tinkerbell. Shortly after, Tinkerbell was removed from this list since she did not fit the Princess mythology.
As a result, Disney launched Tinkerbell's own franchise, The Disney Fairies, of which she is the leader. There are also 4 other fairies in this franchise. They will have their own movie this October, to match the huge retail line that already exists.
So while Tinkerbell was shortly considered a Disney Princess, due to the Princess mythology, she is no longer part of that, and is simply a Disney Fairy.