Commonwealth, n.
Etymology: [Common + wealth well-being.]
1. A state; a body politic consisting of a certain number of men, united, by compact or tacit agreement, under one form of government and system of laws. The trappings of a monarchy would set up an ordinary commonwealth. Milton.
Note: This term is applied to governments which are considered as free or popular, but rarely, or improperly, to an absolute government. The word signifies, strictly, the common well-being or happiness; and hence, a form of government in which the general welfare is regarded rather than the welfare of any class.
2. The whole body of people in a state; the public.
3. (Eng. Hist.)
2 more senses
Specifically, the form of government established on the death of Charles I., in 1649, which existed under Oliver Cromwell and his son Richard, ending with the abdication of the latter in 1659.
Syn. -- State; realm; republic.
Shades of meaning
Fernald has no entry of its own for commonwealth; he lists it under people.
Where he writes about it:
A community is in general terms the aggregate of persons inhabiting any territory in common and viewed as having common interests; a commonwealth is such a body of persons having a common government, especially a republican government; as, the commonwealth of Massachusetts. — from people
A community may be very small; a commonwealth is ordinarily of considerable extent. — from people
In American usage State signifies one commonwealth of the federal union known as the United States. — from people