I have long believed that Enterprise was just an empty craft, with only the instruments to operate in shorts flights in the atmosphere.
Basically a large glider with odd wings.
However, I have been seeing Enterprise referred to as not space-rated or space-ready, and sometimes as an 'orbiter'. (For example Space Shuttle Orbiter Approach and Landing Test: Final Evaluation Report that is linked in @OrganicMarble's answer calls it "Orbiter 101" a few times)
This makes it seem like Enterprise was actually a complete Space Shuttle, but missing some testing or without a few required space elements.
How much of a prototype actually was Enterprise?
It apparently did not have engines, but the other shuttles also needed to refurbish the engines after each flight.