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.
      [1913 Webster]

   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.
         [1913 Webster]

   2. (Meteor.) A rain cloud; one of the four principal
      varieties of clouds. See Cloud.
      [1913 Webster]