Halo n. : 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.

Halo n. : 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.

Halo n. : An ideal glory investing, or affecting one's perception of, an object.

Halo n. : A colored circle around a nipple; an areola.

Halo v. t. & i. : To form, or surround with, a halo; to encircle with, or as with, a halo.

Example:
What is the difference between Halo 2 and Halo 2:limited edition?

Taxonomy
Halo 3, finale, Trilogy.

Defination from the web:
Halo 3 from Bungie Studios. The Master Chief's AI companion Cortana is still trapped in the clutches of the Gravemind as a civil war is raging in the heart of the Covenant.