The classic definition of GT: "Grand Tourer"
The Grand Tourer, or touring car, means different things to different people. The phrase has evolved from a literal "touring car" to encompass virtually all four-door vehicles that are equipped for long drives. Many of today's grand tourers are tributes to a particular automaker's performance in touring races. The invention of the so-called "horseless carriage" at the end of the 19th century allowed people to see areas beyond their communities in safety and comfort, and the automobile's design made touring a lot easier.
Many cars from the first part of the 20th century came without a top, exposing passengers and drivers to the elements. Some came with optional surrey tops, and later models came with canvas tops. Few had any windows at all, and these vehicles were most commonly called touring cars. Up until the middle of the 1920s, coach builders built cars in much the same way they'd designed horse-drawn carriages. The bodies were larger than the typical roadster, with a rear seat for two or two rear benches; some rear seats were elevated.
Most grand tourers from after WWI came with side curtains that could be snapped into position. However, motorists became tired of the cold air seeping in and demanded hardtop vehicles. All major auto builders (GM, Chrysler, Ford, Studebaker and Dodge) listened and they continued to market their automobiles as touring cars. Before the war, most car makers sold their cars as reliable transportation because mechanical issues were part of driving back then. With advances in technology, that strategy was no longer effective, and car makers began to market the adventure of their grand tourers.
However, the cars of that time were no longer tourers in the most strict sense of the word, because enclosing a car offers a restricted view of one's surroundings. By the 1930s, the tourer became less about enjoying the road and more about long-distance comfort. The four-door sedan and the roadster were sold as sports tourers. Marketing cars as grand tourers fell out of vogue in the US in the time after WWII, although touring models were still sold by British car makers. Many European manufacturers sold tourers based on their performances in touring races; the Grand Tourer became popular as a way to identify 2+2 coupes that can travel long distances at high speeds.