I figure Disney should know who they want to call a princess. That is why I went to the official site.
Pocahontas is considered a princess because her father is the chief of the tribe, ie. the “ruler.” As his daughter should would be the equivalent of a princess.
Belle does indeed marry a prince, thus becoming, as Princess Di did, a princess.
Mu Lan comes close to marrying a prince in the sequel, but doesn’t. She is actually the only Disney “Princess” who is neither born nor wed into royalty.
However, little girls don’t pay a lot of attention to details like that.
And @Jack79 – Anastasia was not Disney. The movie was released by 20th century Fox.