"between two rivers" (usually, if not exclusively, with reference to Mesopotamia), 1774, from Late Latin interamnius, from inter "between" (see inter-) + amnis "a river," a word perhaps of Celtic origin (see afanc).
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.
cattle-devouring aquatic monster in Celtic countries, from Celtic *abankos "water-creature," from *ab- "water" (source also of Welsh afon, Breton aven "river," Latin amnis "stream, river," which is believed to be of Italo-Celtic origin), from PIE root *ap- (2) "water" (see water (n.1)).