An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

Prefix meaning "outside; beyond the scope of; in addition to what is used or expected." Also extro-.

M.E., from L. extra, adverb and preposition, "outside, except, beyond," from exter "outward, on the outside."

Ostar-, from os-, → ex-, + -tar comparative suffix (Mid.Pers. -tar; Av. -tara; PIE base *-tero); borun-, from borun "out, the outside" (Mid.Pers. bêron, from bê "outside, out, away" + rôn "side, direction;" Av. ravan- "(course of a) river").