SIAM
n. uncountable C1 Advanced US //ˈsaɪæm// siam
n. the old name for the country we now call Thailand. It was used until the middle of the 20th century.
n. the historical name for the Kingdom of Thailand, used officially until 1939 and briefly again between 1945 and 1949.
My grandfather has an old map that still shows the country as Siam.
Historians studying Southeast Asian trade routes often refer to the region as Siam when discussing the 18th century.
The diplomatic relations between the British Empire and Siam were formalised through various treaties that recognized the kingdom's unique position as a buffer state in the region.
Usage
Used as a proper noun to refer to the historical state; it does not take an article.