ἅλως

The Greek word halos (ἅλως) originally meant 'threshing floor' — a circular, flat surface where grain was separated from chaff. By extension, it came to describe any disk or ring, including the luminous ring sometimes visible around the sun or moon caused by ice crystals in the upper atmosphere. The astronomical halo was the threshing floor of the sky.

Early Christian artists in the 3rd and 4th centuries began painting circular rings of light behind the heads of Christ, the saints, and the angels. The convention was not original — Hellenistic rulers, Roman emperors, and Buddhist figures were depicted with radiant circles before Christianity adopted the device. But the Christian halo became the most persistent version, standardized by the 5th century into the flat golden disk familiar from a thousand icons.

The word halo entered English in the 1560s, initially for the atmospheric phenomenon around the sun and moon. The artistic meaning — the nimbus around a sacred figure's head — followed by the early 1600s. Artists and theologians used different words for different forms: a nimbus was a cloud of light, an aureole was a full-body glow, and a halo was specifically the ring. English collapsed all three into 'halo.'

The metaphorical 'halo effect' — the cognitive bias where one positive trait makes everything about a person seem positive — was named by psychologist Edward Thorndike in 1920. The saint's golden disk became a term in behavioral psychology. A threshing floor became a ring of light became a statistical bias. The word has been spiraling outward from its center for two thousand years.