A “semi truck” is a freight-hauling vehicle consisting of a heavy duty cab/engine attached to a large, multi-axle trailer. The combination of cab and trailer are also sometimes called an eighteen wheeler, a tractor-trailer, a big rig, and, in Great Britain, an “articulated lorry.”
Google Trends data shows that “semi truck” is the most common term for the vehicle across most U.S. states. The exceptions are along the eastern seaboard, where “tractor trailer” is most common, and in the south, where “eighteen wheeler” is frequently uttered.+
Semi trucks actually get their name from semi trailers. Since a semi trailer does not have front wheels and cannot be hauled on its own without a truck cab, it is known as a “semi” trailer rather than a full or complete trailer. Since the specialized cab hauls the semi trailer, it has come to be known as a semi truck.
Semi truck accidents can be particularly dangerous with a trailer attached since the large, heavy apparatus can damage several vehicles at once before coming to rest potentially across multiple lanes of traffic. But even a cab by itself commonly weighs over 20,000 pounds, meaning that an accident that could be a minor fender bender if two passenger vehicles were involved instead turns catastrophic.