Thailand vs Siam

Thailand and Siam both are nouns.

Word	Noun	Adjective	Verb	Adverb
Thailand	Yes	No	No	No
Siam	Yes	No	No	No

As nouns, Thailand and Siam are synonyms defined as:

Thailand and Siam: a country of southeastern Asia that extends southward along the Isthmus of Kra to the Malay Peninsula

Other synonyms of Thailand include Kingdom of Thailand.

Thailand (noun)	Siam (noun)
a country of southeastern Asia that extends southward along the Isthmus of Kra to the Malay Peninsula	a country of southeastern Asia that extends southward along the Isthmus of Kra to the Malay Peninsula