Mustang Cat-back (also cat back and catback) exhaust systems refers to the portion of the exhaust system from the outlet of the catalytic converter to the exhaust tips. This generally includes the pipe from the converter to the muffler, the muffler, and the final length of pipe to open air.
Cat-back exhaust systems are a very popular Ford Mustang aftermarket performance enhancement. Cat-back exhaust systems generally use larger diameter pipe than the stock system. Good cat-back exhaust systems will have mandrel-bent turns that allow the exhaust gas to exit with as little back pressure as possible.
Some people assume that "cat-back" exhaust systems get their name because the hump in the exhaust tubing used to clear the rear axles looks like a cat's back. This is untrue, as explained above the name "cat-back" is given because the system is designed to replace the stock system from the "cat" or catalytic converter, "back" meaning to the rear exhaust tip.