A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, a lake, or another bay.^[1][2][3] A large bay is usually called a gulf, sea, sound, or bight. A cove is a small, circular bay with a narrow entrance. A fjord is an elongated bay formed by glacial action.^[4] The term embayment is also used for related features, such as extinct bays or freshwater environments.

A bay can be the estuary of a river, such as the Chesapeake Bay, an estuary of the Susquehanna River.^[2] Bays may also be nested within each other; for example, James Bay is an arm of Hudson Bay in northeastern Canada. Some large bays, such as the Bay of Bengal and Hudson Bay, have varied marine geology.^{[citation needed]}

The land surrounding a bay often reduces the strength of winds and blocks waves. Bays may have as wide a variety of shoreline characteristics as other shorelines. In some cases, bays have beaches, which "are usually characterized by a steep upper foreshore with a broad, flat fronting terrace".^[5] Bays were significant in the history of human settlement because they provided easy access to marine resources like fisheries.^[6] Later they were important in the development of sea trade as the safe anchorage they provide encouraged their selection as ports.^[7]

The United Nations Convention on the Law of the Sea defines a bay as a well-marked indentation in the coastline, whose penetration is in such proportion to the width of its mouth as to contain land-locked waters and constitute more than a mere curvature of the coast. An indentation, however, shall not be regarded as a bay unless its area is as large as (or larger than) that of the semi-circle whose diameter is a line drawn across the mouth of that indentation — otherwise it would be referred to as a bight.^[8][a]

• Open bay — a bay that is widest at the mouth, flanked by headlands.
• Enclosed bay — a bay whose mouth is narrower than its widest part, flanked by at least one peninsula.
• Semi-enclosed bay — an open bay whose exit is made into narrower channels by one or more islands within its mouth.
  • Back-barrier bay — a semi-enclosed bay separated from open water by one or more barrier islands or spits.^[9]

There are various ways in which bays can form. The largest bays have developed through plate tectonics.^[7] As the super-continent Pangaea broke up along curved and indented fault lines, the continents moved apart and left large bays; these include the Gulf of Guinea, the Gulf of Mexico, and the Bay of Bengal, which is the world's largest bay.^[7]

Bays also form through coastal erosion by rivers and glaciers.^[7] A bay formed by a glacier is a fjord. Rias are created by rivers and are characterised by more gradual slopes. Deposits of softer rocks erode more rapidly, forming bays, while harder rocks erode less quickly, leaving headlands.

• Bay platform – Dead-end railway platform at a railway station that has through lines
• Great capes – Three major capes of the traditional clipper route