Bay leaf (plural bay leaves) refers to the aromatic leaf of the bay laurel (Laurus nobilis, Lauraceae). Fresh or dried bay leaves are used in cooking for their distinctive flavor and fragrance. The leaves are often used to flavor soups, stews, braises and pâtés in Mediterranean cuisine. The fresh leaves are very mild and do not develop their full flavor until several weeks after picking and drying.

The term "bay leaf" is used to refer to several other plants besides the leaves of L. nobilis. These include:

The bay laurel tree has been cultivated since the beginning of recorded history; it originated in Asia Minor, and spread to the Mediterranean and other countries with suitable climates. Bay leaf is not grown in northern regions, as the plants do not thrive in cold climates. Turkey is one of the main exporters of bay leaves, although they are also grown in areas of Albania, France, Belgium, Italy, Russia, Central America, North America, and India. The laurel tree was very important both symbolically and literally in both Greece and Rome. The laurel can be found as a central component in many ancient mythologies that glorify the tree as a symbol of honor. Bay leaves are one of the most widely used culinary herbs in Europe and North America. In the Elizabethan era, some people believed pinning bay leaves to one's pillow on the eve of Saint Valentine's Day would permit one to see one's future spouse in a dream.

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. 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.

There are various ways that bays can be created. The largest bays have developed as a result of continental drift. As the super-continent Gondwana 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, Persian Gulf, Gulf of Alaska, Gulf of Mexico and the Bay of Bengal which is the largest bay in the world.

A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants.

Typically a leaf is a thin, flattened organ borne above ground and specialized for photosynthesis, but many types of leaves are adapted in ways almost unrecognisable in those terms: some are not flat (for example many succulent leaves and conifers), some are not above ground (such as bulb scales), and some are without major photosynthetic function (consider for example cataphylls, spines, and cotyledons).

Conversely, many structures of non-vascular plants, or even of some lichens, which are not plants at all (in the sense of being members of the kingdom Plantae), do look and function much like leaves. Furthermore, several structures found in vascular plants look like leaves but are not actually leaves; they differ from leaves in their structures and origins. Examples include phyllodes, cladodes, and phylloclades.