An island of glacial origin off the southern coast of Massachusetts, U.S., 4 miles across Vineyard Sound from the mainland. It is the 58th largest island in the US, with a land area of about 96 square miles, and the third-largest on the East Coast of the US. The island now has a year-round population of about 15,000 in six towns; in summer, the population increases to 100,000, with more than 25,000 short-term visitors coming and going on during the summer season.