I am definitely not in the trucking business.
But ... semis are called such because they pull "Semi-Trailers." A semi-trailer is a trailer that only has one axel. Semi is actually short for "semi-trailer" truck.
Founder & CEO
A semi is a trailer box with a rear axle but without a front axle. The truck pulling it is called the tractor.
This comes under general knowledge. A semi is correctly a semi-trailer. It doesn't trail behind the rear wheels of the vehicle pulling it. The attachment point is in front of the rear wheels of the tractor. Hence it is not a full trailer, but a semi-trailer. Correctly the rig is known as a tractor/semi-trailer. The reason for this configuration is that a large portion of the load is carried by the tractor and not on the hitch of the trailer attachment.
Semi is short for Semi-Trailer Truck. A semi-trailer is one that does not have front wheels. Instead, the front of the trailer rests on a coupling plate ("fifth wheel") on the tractor unit.
In the U.K., it is called an Articulated Lorry.
John Z Wetmore
Producer of "Perils For Pedestrians" Television
For many years I owned and operated a high rise rigging and heavy hauling trucking company covering the - states of NY, Conn, NJ, Pa, and Va.
Any haulage combination connecting a fifth wheel tractor to a king pin trailer articulating and controlled by the tractor operator managing a quick connect common system of air brakes and running, stop and signaling lights is considered a semi type haulage rig.
Adding to my description the etymology of the word semi meaning half is confusing when associated to the big rigs. There is never a half between a tractor and trailer.
It is at best there could be a one third tractor to two thirds trailer. However, for description purposes the word semi distinguishes a straight single frame type rig from a tractor trailer articulated combo as I described.