(tīʹlănd´, -lənd). Formerly Siam (sī-ămʹ). Abbr. Thai. A country of southeast Asia on the Gulf of Thailand (formerly the Gulf of Siam), an arm of the South China Sea. A Thai state was first established in the region in the mid-14th century but was often dominated by other powers during the succeeding centuries. It became a constitutional monarchy in 1932. Thailand was occupied by the Japanese in World War II and was a strong supporter of the United States in the Vietnam War. Bangkok is the capital and the largest city. Population, 49,515,074, negeri Thai.