An Etymological Dictionary of Astronomy and Astrophysics

A dark space or band between the primary and secondary rainbows when both are visible. This effect is due to the minimum refraction angle for the → primary rainbow and the maximum for the → secondary rainbow. The only light in the dark region is caused by (a small amount of) scattering, and not the refraction of light in water droplets.

See also: Named for Alexander of Aphrodisias, Greek Peripatetic philosopher and commentator, who first described the effect in 200 AD.