A halo or nimbus is often depicted above the head of a holy person. This started as a symbol of solar power or the sun itself and was often depicted over the heads of solar deities. Over time, this symbol came to indicate God’s Light shining forth from within a person.
Halos come in various shapes and sizes and, while some of that is just due to the artist’s choice, different shapes and sizes can actually have specific meanings. A round halo is supposed to be used over the head of a holy person who is dead, physically anyway. A square or hexagon halo is used for a living saint. A square halo can also represent the total Godhead. A triangular halo represents God the Father. A halo around a Phoenix is symbolic of solar power and immortality. A double nimbus represents the dual nature of the world.
In Hindu symbology, the fiery nimbus around Shiva represents the cosmos. In Buddhism, the red halo around Buddha represents solar energy. In Greek and Roman symbology, a blue halo represented a sky or solar god. In ancient Egyptian art, Ra is usually depicted with a solar disk over his head which is essentially a halo. In Christianity, a dove with a three-barred halo represents the holy spirit.
The halo fell out of fashion in western art for centuries. Mystic poet and artist William Blake tried to bring it back by using them in his art, but few other artists followed his example although some say that the lines around his head in some of Van Gogh’s self-portraits are intended to give a hint of a halo as a representation of inner power or fire.