Halo, a circle or arc of light surrounding a luminous body. The name is particularly applied to the luminous circles sometimes seen around the sun or moon. These halos often appear simply as white light, but may show prismatic (rainbowlike) hues. Many halos also include a number of bright arcs and sundogs (bright spots).
Halos are caused by the refraction (bending) and reflection of light by microscopic ice crystals. Cirrostratus clouds, formed of these crystals, cause halos when they pass in front of the sun or moon. Halos often indicate approaching storms, for cirrostratus clouds are usually followed by rain clouds.
In art, a halo is often called a nimbus and has been used by both ancients and moderns to surround the head of a sacred person. In Christian art, the halo is usually circular, but sometimes appears as rays forming a cross. The ancients believed that a holy person's spirituality was visible as a halo.