Why Massachusetts is a Commonwealth
From 1776 to 1780 the words "State of Massachusetts Bay" appeared on the top of all acts and resolves. In 1780, the Massachusetts Constitution went into effect. Part Two of the Constitution, under the heading "Frame of Government" states: "that the people ... form themselves into a free, sovereign, and independent body politic, or state by the name of The Commonwealth of Massachusetts." Virginia (on June 29, 1776) and Pennsylvania (on September 25, 1776) adopted Constitutions which called their respective states commonwealths. Kentucky is also a commonwealth. Commonwealths are states, but the reverse is not true. Legally, we are a commonwealth because the term is contained in the Constitution.
In the era leading to 1780, a popular term for a whole body of people constituting a nation or state (the body politic) was the word "Commonwealth." This term was the preferred usage of some political writers. There also may have been some anti-monarchial sentiment in using the word commonwealth. John Adams utilized this term when framing the Massachusetts Constitution.
Adams wrote: "There is, however, a peculiar sense in which the words republic, commonwealth, popular state, are used by English and French writers, who mean by them a democracy, a government in one centre, and that centre a single assembly, chosen at stated periods by the people and invested with the whole sovereignty, the whole legislative, executive and judicial power to be included in a body or by committees as they shall think proper." (John Adams. Life and Works, vol. 5, p. 454)