The tailpipe is the pipe at the back end of a car, motorbike or machine, where the burnt gas comes out. It is the last part of the exhaust system — the bit you can actually see, sticking out under the back bumper. When a car is cold you sometimes see white steam coming from the tailpipe; black smoke there means something is wrong with the engine. The word is much more common in American English. In Britain people usually say "exhaust pipe", or just "the exhaust". You will meet tailpipe most often in talk about pollution: "tailpipe emissions" means the gas a vehicle puts into the air, and it is the standard phrase in news about clean air rules and electric cars — an electric car is often described as having "zero tailpipe emissions", which means nothing comes out of the back, though the electricity it used may still have been made by burning something. That small distinction is exactly why the word tailpipe is used instead of just "emissions": it limits the claim to the vehicle itself.