An Etymological Dictionary of Astronomy and Astrophysics

A prefix denoting “between, among, mutually, reciprocally.”

Etymology (EN): Inter-, from L. inter (prep., adj.) “among, between,” from PIE *enter “between, among” (cf. Pers. andar-, as below; Skt. antár; Gk. entera (pl.) “intestines,” O.Ir. eter; O.Welsh ithr “among, between;” O.H.G. untar;
O.E. under “under”), from en- “in” + -ter comparative suffix.

Etymology (PE): Mod.Pers. andar “in, into, within” (added to the words pedar “father,” mâdar “mother,” barâdar “brother,” xâhar “sister,” it implies “step-father, step-mother, half-brother, half-sister” respectively), from Mid.Pers. andar “in, among, concerning;” O.Pers. anatr, Av. antarə’ (adv., prep., prev.), antarə (adv., prev.), “inside, intra, within, between;” also Av. antara- (adj.) “interior;” cf. Skt. ántara- “interior, adjacent to; intimate, dear;” from PIE *enter, as above.
The Av. antarə’ is used with verbal stems to create abstract actions. For example,
antarə’-mruyē “to prohibit,” from antare- “inter-” + mruyē “to speak,” from root mrū- “to speak, say.” This is exactly the same pattern as L. inter-dicere “to interdict, forbid, prohibit,” from inter-, as above,

• dicere “to speak.” The Av. antare- is used with another verb to produce the same sense: antarə-uxti “to interdict,” from antarə- + uxti “to speak, to say.” E. Benveniste (1975) made an interesting investigation on the origin of the “to speak inside” paradigm for the concept of interdiction. He argues that inter- derives in fact from *en-ter,
  the second component, while being a comparative form, introduces the notion of separation. His conclusion is that antarə-mruyē
  or inter-dicere mean “to pronounce inside (a group) so as to separate (or isolate somebody).” According to Benveniste, the Av. terms are the oldest forms in the Indo-European languages which convey an important piece of information about an aspect of Indo-European life/tradition in pre-historic times.