The Collaborative International Dictionary of English v.0.48:

Halo \Ha"lo\ (h[=a]"l[-o]), n.; pl. Halos (-l[=o]z). [L.
   halos, acc. halo, Gr. "a`lws a thrashing floor, also (from
   its round shape) the disk of the sun or moon, and later a
   halo round it; cf. Gr. e'ily`ein to enfold, 'ely`ein to roll
   round, L. volvere, and E. voluble.]
   1. A luminous circle, usually prismatically colored, round
      the sun or moon, and supposed to be caused by the
      refraction of light through crystals of ice in the
      atmosphere. Connected with halos there are often white
      bands, crosses, or arches, resulting from the same
      atmospheric conditions.
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   2. A circle of light; especially, the bright ring represented
      in painting as surrounding the heads of saints and other
      holy persons; a glory; a nimbus.
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   3. An ideal glory investing, or affecting one's perception
      of, an object.
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   4. A colored circle around a nipple; an areola.
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