A SEMI is a many-axled vehicle, also known as a SEMI-trailer truck or tractor-trailer. It is a large truck that consists of two parts: the tractor, which is the front part of the truck, and the trailer, which is the back part. The trailer is connected to the tractor by a fifth wheel coupling, and the tractor has three or more axles. SEMI trucks are used for long-distance hauling of goods and materials, and they are an important part of the transportation industry.