A circle of light, emblematical of glory, placed by the old painters round the heads of martyrs and saints. The notion was derived from Exod. xxv. 25. Facies coronam aureolam (“Thou shalt by thine own merits make for thyself a crown, besides that of gold which God has promised to the faithful”) (Donne: Sermons). Strictly speaking, the glory confined to the head alone is a nimbus, and only when it envelops the entire body is it called an aureola.
Du Cange informs us that the aureola of nuns is white, of martyrs red, and of doctors green. The nimbus of a Christ should contain a cross; of the Virgin Mary, a circlet of stars: of God the Father, a triangle with rays; of a living saint, a square without rays.
They say, who know the life divine, And upward gaze with eagle eyne, That by each golden crown on high, Rich with celestial jewelry, Which for our Lord's redeemed is set, There hangs a radiant coronet, All gemmed with pure and living light Too dazzling for a sinner's sight, Prepared for virgin souls, and them Who seek the martyr's diadem.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894