A dakini is a female embodiment of enlightenment energy.  She can appear as a female spirit, witch, yogini, or deity, either human or divine. Since she can appear at any time and at any place, and often disappears quickly, her primary characteristic is her ability to fly. The dakini tradition was part of the goddess tradition of pre-Aryan India.  The Buddhist and tantric traditions conquered, reformed and then appropriated the dakinis and the graveyards where they practiced to new heights of exalted teachings.
Being able to fly is one of the eight common powers (siddhis) that Tibetan masters acquire as they approach enlightenment.  While it takes advanced training to be able to fly, it also takes a certain attainment to be able to see an enlightened being fly.