An Etymological Dictionary of Astronomy and Astrophysics

Prefix meaning “outside; outer; external” used in the formation of compound words: exoplanet, exosphere, exocentric.

Etymology (EN): From Gk. exo “outside,” used in forming scientific terms, → ex-.

Etymology (PE): Borun-, from borun “out, the outside” (Mid.Pers. bêron, from bê “outside, out, away”

• rôn “side, direction;” Av. ravan- “(course of a) river”).