A commonwealth refers to any group of people organized under a single government, particularly a republic. If you live in the United States of America, you live in a commonwealth.
Comprised of the roots common, meaning "belonging to all," and wealth, meaning "happiness or riches," the word commonwealth originally referred to the government of England from 1649 to 1660. Nowadays, a commonwealth has come to mean any government in which all people involved have a say, or a loose formation of nations with a shared loyalty. It is also the official designation of a few states including, Kentucky, Massachusetts, Pennsylvania, and Virginia and some U.S. territories.