Longman Dictionary of Contemporary English
The Collaborative International Dictionary
Inter- \In"ter-\ [L. inter, prep., among, between, a compar. form of in in; akin to intra, intro, within, Skr. antar between, in, and E. in. See In, and cf. Entrails, Interior, Enter-, Exterior.] A prefix signifying among, between, amid; as, interact, interarticular, intermit.
Douglas Harper's Etymology Dictionary
Latin inter (prep., adv.) "among, between, betwixt, in the midst of," from PIE *enter "between, among" (cognates: 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 *en "in" (see in). Also in certain Latin phrases in English, such as inter alia "among other things." A living prefix in English from 15c. Spelled entre- in French, most words borrowed into English in that form were re-spelled 16c. to conform with Latin except entertain, enterprise.
Wiktionary
pre. (non-gloss definition: among, between, amid, during, within, mutual, reciprocal)