The aureola, when enveloping the whole body, generally appears oval or elliptical in form, but occasionally circular or quatrefoil. When it appears merely as a luminous disk round the head, it is called specifically a nimbus, while the combination of nimbus and aureole is called a glory. The strict distinction between nimbus and aureole is not commonly maintained, and the latter term is most frequently used to denote the radiance round the heads of saints, angels or persons of the Godhead.
The nimbus in Christian art appeared first in the 5th century, but practically the same device was known still earlier, though its history remains obscure, in non-Christian art. Thus (though earlier Indian and Bactrian coins do not show it) it is found with the gods on some of the coins of the Indian kings Kanishka, Huvishka and Vasudeva, 58 B.C. to A.D. 41 (Gardner's Cat. of Coins of Greek and Scythic Kings of Bactria and India, Brit. Mus. 1886, plates 26-29). And its use has been traced through the Egyptians to the Greekss and Romanss, representations of Trajan (arch of Constantine) and Antoninus Pius (reverse of a medal) being found with it.
In the circular form the nimbus constitutes a natural and even primitive use of the idea of a crown, modified by an equally simple idea of the emanation of light from the head of a superior being, or by the meteorological phenomenon of a halo. The probability is that all later associations with the symbol refer back to an early astrological origin (compare Mithras), the person so glorified being identified with the sun and represented in the sun's image; so the aureole is the Hvareno of Mazdaism. From this early astrological use the form of "glory" or "nimbus" has been adapted or inherited under new beliefs.