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.
      [1913 Webster]

   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.
      [1913 Webster]

   3. An ideal glory investing, or affecting one's perception
      of, an object.
      [1913 Webster]

   4. A colored circle around a nipple; an areola.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Halo \Ha"lo\, v. t. & i. [imp. & p. p. Haloed (-l[=o]d); p.
   pr. & vb. n. Haloing.]
   To form, or surround with, a halo; to encircle with, or as
   with, a halo.
   [1913 Webster]

         The fire
         That haloed round his saintly brow.      --Southey.
   [1913 Webster] halobacter

WordNet (r) 3.0 (2006):

halo
    n 1: an indication of radiant light drawn around the head of a
         saint [syn: aura, aureole, halo, nimbus, glory,
         gloriole]
    2: a toroidal shape; "a ring of ships in the harbor"; "a halo of
       smoke" [syn: ring, halo, annulus, doughnut, anchor
       ring]
    3: a circle of light around the sun or moon

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "halo":
   O, annular muscle, annulation, annulus, anthelion, antisun, areola,
   aura, aureola, aureole, blaze of glory, brilliance, brilliancy,
   chaplet, charisma, circle, circuit, circumference, circus,
   closed circle, corona, coronet, countersun, crown, cycle, diadem,
   disc, discus, disk, envelope, eternal return, fairy ring, garland,
   glamour, glory, illustriousness, lasso, logical circle, loop,
   looplet, lunar corona, lunar halo, luster, magic, magic circle,
   mock moon, mock sun, moon dog, mystique, nimbus, noose,
   numinousness, orbit, paraselene, parhelic circle, parhelion,
   radiance, radius, rainbow, resplendence, resplendency, ring,
   rondelle, round, roundel, saucer, solar corona, solar halo,
   sphincter, splendor, sun dog, vesica, vicious circle, wheel,
   wreath



The Devil's Dictionary (1881-1906):

HALO, n.  Properly, a luminous ring encircling an astronomical body,
but not infrequently confounded with "aureola," or "nimbus," a
somewhat similar phenomenon worn as a head-dress by divinities and
saints.  The halo is a purely optical illusion, produced by moisture
in the air, in the manner of a rainbow; but the aureola is conferred
as a sign of superior sanctity, in the same way as a bishop's mitre,
or the Pope's tiara.  In the painting of the Nativity, by Szedgkin, a
pious artist of Pesth, not only do the Virgin and the Child wear the
nimbus, but an ass nibbling hay from the sacred manger is similarly
decorated and, to his lasting honor be it said, appears to bear his
unaccustomed dignity with a truly saintly grace.