The term 18-wheeler is a colloquial name for a large truck, typically used to transport goods over long distances. The crossword solutions for this term are RIG (3 letters), SEMI (4 letters), and BIGRIG (6 letters). RIG stands for 'rigid vehicle, ' which is a type of truck with a single axle. SEMI stands for 'SEMI-trailer truck, ' which is a type of truck with two axles and a trailer attached. BIGRIG is a slang term for a large truck, usually referring to an 18-wheeler.