I think it was the 'council of chalcedony' or the 'council of Nicaea' - I forget which. I believe eastern Christianity pertains to Greek orthodoxy (including what would become eastern Europe), whereas oriental Christianity pertains to Syriac, Assyrian, Armenian, Georgian, and possibly Ethiopian). I forget which group north african and coptic belongs to (probably eastern).
The details are fuzzy because I read the book (below) a while back, but it's something to that effect.
What are the Prime Differences between Eastern Orthodoxy and Oriental Orthodoxy?
