word-forming element used freely in English, "between, among, during," from Latin inter (prep., adv.) "among, between, betwixt, in the midst of" (also used extensively as a prefix), from PIE *enter "between, among" (source also of Sanskrit antar, Old Persian antar "among, between," Greek entera (plural) "intestines," Old Irish eter, Old Welsh ithr "among, between," Gothic undar, Old English under "under"), a comparative of root *en "in."
A living prefix in English from 15c. and used with Germanic as well as Latinate words. Spelled entre- in French; most words borrowed into English in that form were re-spelled 16c. to conform with Latin except entertain, enterprise. In Latin, spelling shifted to intel- before -l-, hence intelligence, etc.
penetration n.
early 15c., penetracioun, "a puncture, a penetrating wound," from Latin penetrationem (nominative penetratio) "a penetrating or piercing," noun of action from past-participle stem of penetrare "to put or get into, enter into" (see penetrate). From c. 1600 as "insight, discernment, shrewdness;" the sexual sense is attested from 1610s; meaning "act of penetrating or piercing" is from 1620s; in optics, by 1799.