In American English, a semi-trailer is a trailer without a front axle. A large proportion of its weight is 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 is normally equipped with legs which can be lowered to support it when it's uncoupled. A road tractor coupled to a semi-trailer is often called a semi-trailer truck or semi. Though most road trailers meet this definition, the term is most often appl… (More on Semi-trailer)