IMO, a street car is a car that is legal to drive on the street in what ever state it lives in.
That means passing inspection of applicable, plates, vin, dot tires, seat belts, head lights, turn signal, wipers etc. it must also be able to cruise around in traffic with out over heating.

Other than being legal and able to function like a regular car, I say it's what ever the owner wants to do and is willing to put up with.