China vs mainland China

China and mainland China both are nouns.

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

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

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

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

China (noun)	mainland China (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
a government on the island of Taiwan established in 1949 by Chiang Kai-shek after the conquest of mainland China by the Communists led by Mao Zedong