The Collaborative International Dictionary of English v.0.48:
Nimbus \Nim"bus\, n.; pl. L. Nimbi, E. Nimbuses. [L., a rain
storm, a rain cloud, the cloudshaped which enveloped the gods
when they appeared on earth.]
1. (Fine Arts) A circle, or disk, or any indication of
radiant light around the heads of divinities, saints, and
sovereigns, upon medals, pictures, etc.; a halo. See
Aureola, and Glory, n., 5.
Note: "The nimbus is of pagan origin." "As an attribute of
power, the nimbus is often seen attached to the heads
of evil spirits." --Fairholl.
2. (Meteor.) A rain cloud; one of the four principal
varieties of clouds. See Cloud.