Okay, I'm using Google now to confirm my thoughts.
First, it seems that "rollover" is one word and is used to describe a specific type of accident, often associated with SUVs with their higher center of gravity that took a turn too quickly.
It seems you can flip your car without having what's defined as a "rollover" accident, but that most rollovers will result in vehicles being upside down.
I'm not sure there is a bright line dividing the two, though.