HDT stands for "Heavy Duty Truck," which refers to a type of vehicle designed to haul heavy loads and perform demanding tasks. There are several synonyms for HDT that are commonly used in the transportation industry, such as "big rig," "semi-truck," "tractor-trailer," and "18-wheeler." These terms all refer to the same type of vehicle, but they may be used in different regions or contexts. For example, "big rig" is a term more commonly used in the western United States, while "semi-truck" is used more often in the eastern US. Understanding these synonyms can help improve communication and clarity in the transportation industry.