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 is 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 applied to heavy trailers appropriate for use in such a rig.

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).

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