The noun superpower is good for talking about a very powerful country, especially one that's strong enough to influence the way other countries act.
When one country dominates the world politically, that country is called a superpower. The United States is considered to be a superpower, and throughout most of the twentieth century, so was the Soviet Union. Political scholars have their eye on China to emerge as the next superpower in the world. Officially, one step lower than a superpower is a great power, which has slightly less influence in the world.