The Tappan Zee is a natural widening of the Hudson River, about 3 miles (4.8 km) across at its widest, in southeastern New York. It stretches about 10 miles (16 km) along the boundary between Rockland and Westchester counties, downstream from Croton Point to Irvington. It derives its name from the Tappan people of the Lenape, and the Dutch word zee, meaning a sea.

The Hudson River is a 315-mile (507 km) river that flows from north to south primarily through eastern New York, United States. It originates in the Adirondack Mountains of upstate New York at Henderson Lake in the town of Newcomb, and flows southward through the Hudson Valley to the New York Harbor between New York City and Jersey City, eventually draining into the Atlantic Ocean at Upper New York Bay. The river serves as a physical boundary between the states of New Jersey and New York at its southern end. Farther north, it marks local boundaries between several New York counties. The lower half of the river is a tidal estuary, deeper than the body of water into which it flows, occupying the Hudson Fjord, an inlet that formed during the most recent period of North American glaciation, estimated at 26,000 to 13,300 years ago. Even as far north as the city of Troy, the flow of the river changes direction with the tides.