My opinion is that a supercar does super things. It can get to 60 in a couple of seconds, brake from 100 in a few dozen feet, hold near a g or more on a skidpad, and boogie around the Nurburgring, which is the defacto supercar proving grounds.
...and it has to have a million horsepower, two foot wide tires, scoops, and ducts and vents, and spoilers, and wings with extra absorbancy, and a knee-high roofline....