If you're talking about magical powers, then that would be materia, just like everybody else in the game, though after he falls into the lifestream during the nibelheim incident, it's never confirmed whether or not he actually needs materia to cast the less unique spells (i.e. he still needs materia to cast meteor, but nothing else can be confirmed) although when cloud hands it over in the northern crater it does appear as though sephiroth absorbs the black materia, so he may have done this with other materia also. Other powers such as telekinesis are from an unknown source, but may either be from his higher than normal concentration of jenova cells or a materia unavailable to the party.