What is a muscle car

I would say a certain H.P. per weight of the car, but there's so many 12, 13 and 14 sec. cars nowadays, V6 Mustangs, Mercedes, SUV's, whatever..... I think the whole "muscle car" thing more refers to the "good ol' days" with a hot V8/RWD American metal. When anyone or company refers to their new cars as muscle cars, I think it's just a little bit of a nod to the past.