Originally, it was a "hex" power. She basically altered probability to cause something bad or unlucky to happen to people.
But then later, I think they said her mutant power was to actually tap into magic power, so she was able to use magic without ever having trained, but as a result was just sloppy at it
Then she actually got trained to wield magic properly.
And nowadays...is she even still a mutant, or just a straight up witch now?