a city on the Mississippi River in the US state of Missouri. It was established in 1764 by the French, who named it after King Louis IX. It was part of the Louisiana Purchase and became a major port for steamboats (= river boats driven by steam) in the 19th century. It now produces cars, aircraft, metal and chemicals. The city was known as the 'Gateway to the West', and its Gateway Arch (630 feet/192 metres high) is a symbol of this.