inter-, intero-
(Latin: between; among, mutually, together; on the inside, internal)
Although abstracted from the many compounds in which it entered English, the form inter- was not generally considered a living prefix in English until the 1400s.
During the later period of Middle English many words borrowed in the Old and Middle French forms entre-, enter- began to be consciously respelled with Latin inter-; although vestiges of the older French borrowings are found in entertain and enterprise.
The living prefix inter- is now freely added to almost any element in English to create such formations with the meaning of "between" and "among". The words formed by intra- are closely related to this inter- prefix; in fact, they both apparently came from the same Latin source.