a. A circle of light, either white or prismatically coloured, seen round a luminous body and caused by the refraction of light through vapour; spec. that seen round the sun or moon, commonly of 22 or 46 degrees radius, with the red extremity of the spectrum inside the circle.The definite size of halos and the arrangement of their prismatic colours distinguish them from coronæ, which are phenomena of diffraction, varying in size and having the red outside: see corona n.¹ 1. But the two words are often treated as synonymous.

b. Applied to other circular luminous appearances; hence, by extension, to other things in the form of a circle or ring.

c. A coloured circle, such as those around the nipples, and those which surround vesicles or pustules; = areola n. 3.

d. plural. The rings of lighter and darker colour, usually concentric, in the yolk of an egg, the result of its deposition in successive layers.

e. A style in women's hats (worn at the back of the head with the brim thus framing the face). Also attributive and in other combinations, as halo-brim(med), hat, style, etc.

f. A more or less circular bright or dark area formed in various photographic processes (see quots.).

halo-zone n.

halo-girt adj.

halo-like adj.