A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, lake, or another bay. A large bay is usually called a gulf, sea, sound, or bight. A cove is a type of smaller bay with a circular inlet and narrow entrance. A fjord is a particularly steep bay shaped by glacial activity.
Bays can be the estuary of a river, such as the Chesapeake Bay, an estuary of the Susquehanna River. 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 the Hudson Bay, have varied marine geology.
The land surrounding a bay often reduces the strength of winds and blocks waves. Bays were significant in the history of human settlement because they provided safe places for fishing. Later they were important in the development of sea trade as the safe anchorage they provide encouraged their selection as ports.
The United Nations Convention on the Law of the Sea (UNCLOS), also called the Law of the Sea, defines a bay as a well-marked indentation 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 shall not, however, 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.
There are various ways from which bays can be created. The largest bays have developed as a result of plate tectonics. As the super-continent Pangaea broke up along curved and indented fault lines, the continents moved apart and the world's largest bays formed. These include the Gulf of Guinea, Gulf of Mexico and the Bay of Bengal, which is the largest bay in the world.
Another way bays form is via glacial and river erosion. A bay formed by a glacier is a fjord. Rias are created by rivers and are characterised by more gradual slopes. Currents can make waves more constant, and soft rocks speed erosion. Hard rock eroded less quickly, leaving headlands. The Gulf of California is an example of a bay created by plate tectonics as Baja California peninsula moves away from the Mexican mainland.