Mahācīna becomes current in Buddhist tantric materials of the late first - early second millennium and
perhaps indifferently refers to China, Tibet and neighboring regions.

The derivation from the name of the Qin dynasty has been contested. I believe that Endymion Wilkinson's
Manual of Chinese History has something on this:
http://www.amazon.com/Chinese-History-Harvard-Yenching-Institute-Monograph/dp/0674067150