A bay is a large body of water connected to an ocean or sea formed by an inlet of land due to the surrounding land blocking some waves and often reducing winds. Bays also exist as an inlet in a lake or pond, such as those found in otherwise land-locked areas like Wisconsin and other Great Lake states. A large bay may be called a gulf, a sea, a sound, or a bight. A cove is a circular or oval coastal inlet with a narrow entrance; some coves may be referred to as bays.

Bays were significant in the history of human settlement because they can provide a safe place for fishing. Later they were important in the development of sea trade as the safe anchorage they provide encouraged their selection as ports. Any bay may contain fish and other sea creatures or be adjacent to other bays. For example, James Bay is adjacent to Hudson Bay. Large bays, such as the Bay of Bengal and the Hudson Bay, have varied marine geology.