Ask ten gearheads to define ‘muscle car’ and you’ll get ten different answers — and at least one argument about whether a Mustang counts. The real definition comes down to a specific formula Detroit stumbled into in the mid-1960s: a mid-size body paired with the biggest engine a manufacturer had to offer. From the Pontiac GTO’s accidental invention of the category to a modern restomod chasing genuine ZL1 power, read on to see what actually separates a muscle car from every other fast classic.