http://en.wikipedia.org/wiki/Semi-trailer_truck
A
semi-trailer does not trail completely behind the towing vehicle, but is attached at a point that is just forward of the rear-most axle of the towing unit, so that some fraction of the weight of the trailer is carried by the prime mover, with most of that at the rear axle(s) and a small amount at the front axle(s) of the prime mover.
A semi is sometimes the name of a rig designed to haul semi-trailers.