http://en.wikipedia.org/wiki/Semi-trailer

A semi-trailer is a trailer without a front axle. A large proportion of its weight is supported by a road tractor, by a detachable front axle assembly known as a dolly, or by the tail of another trailer. A semi-trailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled.

A road tractor coupled to a semi-trailer is often called a semi-trailer truck or semi, or in the UK an articulated lorry.

In Australian English, the tractor unit is usually referred to as a prime-mover; and the combination of a prime-mover and trailer is known as a semi-trailer or semi. Semi-trailers with two trailer units are B-Doubles or road trains. A B-double consists of a prime mover towing two semi-trailers, where the first semi-trailer is connected to the prime mover by a fifth wheel coupling and the second semi-trailer is connected to the first semi-trailer by a fifth wheel coupling. A road train means a combination, other than a B-Double, consisting of a motor vehicle towing at least two trailers (counting as a single trailer a converter dolly supporting a semi-trailer).