A semi-trailer is a trailer without a front axle. Also called semi, a detachable trailer for hauling freight, with wheels at the rear end, the forward end being supported by the rear of a truck tractor when attached. one or more semi-trailers are known as “semis”, “semi-trailers”, “tractor-trailers”, “big rigs”, “semi-trucks”, “18-wheelers”, or “semi-tractor trailers” and “semi-trailer trucks”.