Flying cars don't have a well established definition because they aren't a well established technology. That said if you wanted to define them you should define them as a subset of planes. They are planes by definition but defining them as just plane is not useful. A flying car is subset of planes that can also function as a normal car. If you have a plane that can drive to the shops on wheels and park in a parking lot then it becomes a flying car. You can't park a standard blimp like a car, you can't drive it down a highway, so it is not a flying car.