communist country
Refers to a nation governed under communist principles. Often used in political or historical contexts.
China is a communist country.
The Soviet Union was once a powerful communist country.
共産主義 is the ideology (communism), while 共産主義国 is a country that follows that ideology.
社会主義国 (socialist country) is a broader term; many communist countries are also socialist, but the terms are not identical.
Compound of 共産主義 (communism) + 国 (country). The term is a straightforward modern political coinage.