The defining quality of a semi-trailer

Differing opinion here:

The term for the whole assemblage is tractor-trailer. The tractor is the cab/motor stuff up front that supplies the motive power. The trailer is the part that trails behind (duh!). The diference between a full up trailer and a semi-trailer is that the regler trailer has a full set of functional support wheels, fore and aft. A semi-trailer has only rear wheels, relying on the tractor for front end support during travel. There is a set of vestigal looking supports that can be used when the semi-trailer is sitting around not going anywhere waiting for a tractor but they wouldn't be much use while barreling down the highway. Sometimes the term semi is used to describe the whole thing.