Nan·tuck·et (nān-tŭk'ĭt) An island of southeast Massachusetts south of Cape Cod, from which it is separated by Nantucket Sound, an arm of the Atlantic Ocean. Settled in 1659, the island was part of New York from 1660 to 1692, when it was ceded to Massachusetts. It was a whaling center until the mid-1850s and is now a popular resort. Nan·tuck'et·er n.