Why did I always think of the Urals as the terminus of eastern europe?
IMO that's as good a choice as any. If you're going to geographically/ecologically make the distinct entity known as Europe, which, as mentioned, was already referred to as such in acncient times, than looking for obvious termini are your best choice, since they will be the best barriers to ecological/cultural mixing. To the north, south, and west you have large bodies of water that, with the exception of the British Isles, pretty much isolate continental Europe. Its the east that is tricky, and since mountains are better than rivers at dividing ecologies and peoples I would have to say that the Urals and Cauacasus mountain ranges serve as good termini. Though I prefer to think of it primarily as Eurasia rather than Europe and Asia. No need to add Africa in since its climate is so much different from much of Eurasia as to make ecological mixing moot, even if cultural mixing obviously occured.
Anyway, just my 2 cents.