1560s, from Latin halo (nominative halos), from Greek halos "disk of the sun or moon, ring of light around the sun or moon" (also "threshing floor" and "disk of a shield"), of unknown origin. Sense of "light around the head of a holy person or deity" first recorded 1640s. As a verb from 1801.
halo ha·lo (hā'lō)
n. pl. ha·los or ha·loes
A reddish yellow ring surrounding the optic disk, caused by an expansion of the scleral ring that makes the deeper structures visible.
A ring of light surrounding a luminous body.