Aureole

The golden halos around the heads of pagan gods and Christian saints refer both to their being bathed in the glory of the sun and also to the fact that a spiritual sun within their own natures is radiating its glow-ray and surrounding them with celestial splendor. Whenever the nimbus is composed of straight radiant lines, it is solar in significance; whenever curved lines are used for beams, it partakes lunar nature; whenever they are united, it symbolizes a, harmonious blending of both principles. The circular nimbus is solar and masculine, while the lozenge-shaped nimbus, or vesica piscis, is lunar and feminine. The same symbolism is preserved in the circular and lozenge-shaped windows of cathedrals. There is a complete science contained in the shape, color, and adornments of the halos of saints and martyrs. A plain golden ring usually surrounds the head of a canonized saint, while God the Father and God the Son have a far more ornate aureole, usually adorned with a St. George Cross, a flowered cross, or a lilied cross, with only three of the arms visible.

From Audsley's Handbook of Christian Symbolism.