mainland China vs Cathay

mainland China and Cathay both are nouns.

Word	Noun	Adjective	Verb	Adverb
mainland China	Yes	No	No	No
Cathay	Yes	No	No	No

As nouns, mainland China and Cathay are synonyms defined as:

mainland China and Cathay: a communist nation that covers a vast territory in eastern Asia; the most populous country in the world

Other synonyms of mainland China include China, Communist China, People's Republic of China, PRC, Red China.

mainland China (noun)	Cathay (noun)
a communist nation that covers a vast territory in eastern Asia; the most populous country in the world	a communist nation that covers a vast territory in eastern Asia; the most populous country in the world