Merriam-Webster Collegiate Dictionary - commonwealth

 noun  Date: 15th century  1. archaic commonweal 2  2. a nation, state, or other political unit: as  a. one founded on law and united by compact or tacit agreement of the people for the common good  b. one in which supreme authority is vested in the people  c. republic  3. capitalized  a. the English state from the death of Charles I in 1649 to the Restoration in 1660  b. protectorate 1b  4. a state of the U.S. — used officially of Kentucky, Massachusetts, Pennsylvania, and Virginia  5. capitalized a federal union of constituent states — used officially of Australia  6. often capitalized an association of self-governing autonomous states more or less loosely associated in a common allegiance (as to the British crown)  7. often capitalized a political unit having local autonomy but voluntarily united with the U.S. — used officially of Puerto Rico and of the Northern Mariana Islands