The term 'trailer truck' refers to a large vehicle used for transporting goods. It is composed of two parts: a tractor unit, also known as a 'RIG' (3 letters), and a trailer, also known as a 'SEMI' (4 letters). The RIG is the front part of the vehicle, which is usually powered by a diesel engine, while the SEMI is the back part, which is designed to carry cargo. Together, they form a powerful and efficient means of transportation.