A halo is an area of light, usually circular, around the head of a holy figure in religious painting; it is also associated with some of the Classical gods and goddesses, such as Apollo, who sometimes has rays of light around his head.
The halo in Christian art first appears in the 5th century, originally for the three members of the Trinity and for angels. It was subsequently applied to the Virgin Mary and other saints, and even living figures, in Byzantine art.
A halo with a cross inscribed in it (or cruciform aureole) denotes Christ; a triangular halo denotes God the Father.