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.
c. 1300, relacioun, "relationship, connection, correspondence;" late 14c. as "act of telling or relating in words," from Anglo-French relacioun, Old French relacion "report, connection" (14c.) and directly from Latin relationem (nominative relatio) "a bringing back, restoring; a report, proposition," from relatus (see relate).
The meaning "person related by blood or marriage" is attested from c. 1500. The phrase no relation "not in the same family," used in differentiating persons with the same surname, is attested by 1930.