Fr.: lentille convexo-concave
A diverging lens having one surface convex and the opposite surface concave.
Prefix meaning "outside; outer; external" used in the formation of compound words: exoplanet, exosphere, exocentric.
From Gk. exo "outside," used in forming scientific terms, → ex-.
Borun-, from borun "out, the outside" (Mid.Pers. bêron, from bê "outside, out, away" + rôn "side, direction;" Av. ravan- "(course of a) river").