Massachusetts was a state of the United States of America.
History
Massachusetts was one of the thirteen British colonies prior to the American War of Independence. The First Doctor, Susan, Ian and Barbara visited Massachusetts in 1692. (PROSE: The Witch Hunters)
During the American Civil War, Claire Bartlett lived in Diensberg from which she sent several letters to her lover Will Johnson. Johnson hoped to settle in Diensberg with Bartlett after the war and to become the manager of that town's steelworks. (PROSE: Blood and Hope)
In the late 20th century, Space Control received tracking information from an unknown authority within the state. (TV: The Ambassadors of Death)
Other information
Janet Rae Tanner's parents Gerald and Elaine lived in Nantucket in the late 20th and/or early 21st century. (TV: The Middle Men)
Behind the scenes
MIT, mentioned in TV: The Custodians, presumably refers to the Massachusetts Institute of Technology located in Cambridge, Massachusetts. Harvard University is also located in Cambridge.