In American English a semi-trailer's a trailer without a front axle. A large proportion of its weight's supported either by a road tractor or by a detachable front axle assembly known as a dolly or by the tail of another trailer. A semi-trailer's normally equipped with legs which can be lowered to support it when it's uncoupled. A road tractor coupled to a semi-trailer's often called a semi-trailer truck or semi. Though most road trailers meet this definition, the term's most often applied to h… (More on Semi-trailer)