Long Island is a densely populated island off the East Coast of the United States, beginning at New York Harbor just 0.35 miles (0.56 km) from Manhattan Island and extending eastward into the Atlantic Ocean. The island comprises four counties in the U.S. state of New York: Kings and Queens counties (which comprise the New York City boroughs of Brooklyn and Queens, respectively) in the west, and Nassau and Suffolk counties in the east. However, many people in the New York metropolitan area (even those living in Brooklyn and Queens) colloquially use the term “Long Island” (or “The Island”) to refer exclusively to Nassau and Suffolk counties, which are mainly suburban in character. The majority of New York City residents now live on Long Island.

Broadly speaking, “Long Island” may refer both to the main island and the surrounding outer barrier islands. North of the island is Long Island Sound, across which lie Westchester County, New York, and the state of Connecticut. Across the Block Island Sound to the northeast is the state of Rhode Island. To the west, Long Island is separated from the Bronx and the island of Manhattan by the East River. To the extreme southwest, it is separated from Staten Island and the state of New Jersey by Upper New York Bay, the Narrows, and Lower New York Bay. To the east lie Block Island and numerous smaller islands.

Both the longest and the largest island in the contiguous United States, Long Island extends 118 miles (190 km) eastward from New York Harbor to Montauk Point, with a maximum north-to-south distance of 23 miles (37 km) between Long Island Sound and the Atlantic coast. With a land area of 1,401 square miles (3,630 km²), Long Island is the 11th-largest island in the United States and the 149th-largest island in the world—larger than the 1,214 square miles (3,140 km²) of the smallest U.S. state, Rhode Island.