From AMS Glossary
Any of a family of colored or whitish rings, arcs, pillars or spots of light that appear in the sky and are explained by the reflection or refraction of light by ice crystals.
They are usually found in the vicinity of the light source, the most important of which are the sun and moon, but may arise from artificial lights if seen, say, through an ice fog. Halos exhibiting some prismatic coloration are explained, at least in part, by the refraction of light by the ice crystals. However, the color is usually fairly pale, being best on a red edge next to the light. The exceptions, in having very good colors, are the circumhorizontal and circumzenithal arcs, the positions of which are not determined by the minimum angle of refraction. Halos that are white, or show the same color as the light source itself, are explained by the reflection of light off the crystal faces. Whether explained by reflection or refraction, the pattern that emerges depends upon the crystal type, crystal orientation (actually the probability of various orientations within a population of crystals), and the elevation angle of the light (sun). With such a rich range of possibilities, a large variety of halos are theoretically possible and over 50 different halo phenomena have been documented photographically. Some halos predicted theoretically have yet to be reported, others that have been reported have yet to be explained. The most common halo is the halo of 22°. Other frequently seen halos are the parhelia of the halo of 22°, the sun pillar, the 22° tangent arcs, the circumzenithal arc, the halo of 46°, and the parhelic circle. On very rare occasions an observer's sky will be filled with a display with 10, 20, or more different halos, usually persisting for only a few minutes. Much supernatural lore has been prompted by observations of old of such events. Halos must be distinguished from optical phenomena arising from water drops, such as the rainbow, corona, and glory.