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The official name of the state is the "Commonwealth of Massachusetts".[17] Colloquially, it is often referred to simply as "the Commonwealth". While this designation is part of the state's official name, it has no practical implications. Massachusetts has the same position and powers within the United States as other states.[1
http://en.wikipedia.org/wiki/Massachusetts

Massachusetts, like Pennsylvania, Virginia, and Kentucky, is called a "Commonwealth". Commonwealths are states, but the reverse is not true. Legally, Massachusetts is a commonwealth because the term is contained in the Constitution. In the era leading to 1780, when the state Constitution was ratified, a popular term for a whole body of people constituting a nation or state was the word "Commonwealth". This term was the preferred usage of some political writers. There also may have been some anti-monarchic sentiment in using the word "Commonwealth". The name, which in the eighteenth century was used to mean "republic", can be traced to the second draft of the state Constitution, written by John Adams and accepted by the people in 1780. In this second draft, 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". The people had overwhelmingly rejected the first draft of the Constitution in 1778, and in that draft and all acts and resolves up to the time between 1776 and 1780, the name "State of Massachusetts Bay" had been used. Thereafter, John Adams utilized the term "Commonwealth" when framing the Massachusetts Constitution. In his "Life and Works", 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".
http://www.sec.state.ma.us/cis/cismaf/mf1a.htm