Ooh toughie!
Erm, ok here's my stab at it:
A supercar has to be expensive, fast, turn heads (everybody's, not just car enthusiasts), and IMO most important, not share body or chassis parts with any other models lower in the range. Being mid-engined isn't a requisite (like some of you seem to imply looking at you, Aqula DK) but it certainly helps. That means things like the Z-Tune - and R34 GTR for that matter - and BMW M6 can't be classified as a supercar because they share majority of parts and especially styling with lesser models.
Other than that, it's really quite hit-and-miss about what is and isn't a supercar. For me my bottom rung would be the Audi R8 and Honda NSX, then moving up to the F430, Gallardo, DB9 and 911 Turbo. The upper ceiling of "supercar" would end at the really high end stuff like the Vanquish, DBS, 599GTB, and regular Murcielago. Things like the Zonda, Koenigsegg, Carrera GT, Enzo and Veyron are, as I've already said, "hypercars".