A halo (byāmappadhā or pabhāmaṇḍala) is a ring of light believed to appear around the head of particularly saintly people. Ancient and modern depictions of the Buddha often show him with a halo although the Tipitaka never mentions him having one.
However, it does say at the time he attained enlightenment rays (raṃsi) of blue, yellow, red, white, orange light emanated from his body (Vin.I,25). Also, in the hours before his passing his skin became extraordinarily clear and bright (D.II,134). Exactly how this type of transfiguration happens is a mystery, but it is a phenomena mentioned in cultures widely separated from each other.