The small bay, deep inland and sheltered from the wind, has its own hydrological regime.

A bay is a part of an ocean, sea, lake or other body of water that extends deep into the land but has a free exchange of water with the main body of water.

Bays include bays (seas and lakes), inlets (often river mouths), estuaries, fjords, mouths, lagoons (usually along sandy shores), harbours, etc.

The hydrological and hydrochemical conditions of a bay are identical to those of the water body to which it belongs. In some cases, the local climate and inland currents may give some specific characteristics to the hydrological characteristics of the surface layer of the bays.

The largest bays in the world's oceans include the Gulf of Alaska, Bengal, Bay of Biscay, Great Bay, Australia and Guinea.

In some cases, the name 'gulf' has been reserved for waters that are seas in terms of their hydrological regime (e.g. the Mexican, Hudson, Persian, Californian).

Bays are also commonly referred to as submerged craters of volcanoes and lagoons of atolls.

As defined in international law by the 1982 United Nations Convention on the Law of the Sea, "a bay is a well-defined depression in the shore which, in relation to the width of its entrance, extends so far into the land as to contain land-locked waters and constitutes something more than a mere curve in the shore. However, a depression is not considered to be a bay unless its area is equal to or greater than the area of the semicircle whose diameter is the line passing through the entrance to the depression. In other words, the water area of a bay must not be less than the area of a semicircle with a diameter equal to the width of the entrance. If a depression has several entrances due to islands, the width of the entrance is the sum of the widths of the individual entrances. The area of the islands within the entrance is added to the water area. The area of the bay and the widths of the entrances are measured from the low water line.