Definition 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.

: any of several shrubs or trees (such as the red bay or sweet bay) resembling the laurel compare bay rum

Hudson Bay, Canada, is the largest bay in North America, and the largest by by shoreline in the world. It is also the namesake of the oldest corporation on the continent.

Idiom bay for blood (Definition of bay from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)

Bay, concavity of a coastline or reentrant of the sea, formed by the movements of either the sea or a lake. The difference between a bay and a gulf is not clearly defined, but the term bay usually refers to a body of water somewhat smaller than a gulf.

A bay is fundamentally a recessed, coastal body of water that is partially enclosed by land. This feature always maintains a direct, wide opening to a larger main body of water, such as an ocean or a sea, which distinguishes it from more restrictive inlets.

A bay is a partly enclosed area, inside or outside a building, that is used for a particular purpose. The animals are herded into a bay, then led into the barn. The car reversed into the loading bay.