Averrhoa bilimbi, commonly known as the bilimbi, cucumber tree, or tree sorrel, is a tropical, perennial fruiting tree in the family Oxalidaceae. It is native to the warm, humid regions of Southeast Asia, specifically Malaysia and Indonesia, and has been widely naturalized across tropical climates, including South America, the Caribbean, and India. The genus name Averrhoa honors the 12th-century Andalusian philosopher and physician Ibn Rushd (Averroes), while the species epithet bilimbi is derived from its common name in the Malabar coast of India. Historically, the bilimbi has been a staple in tropical agroforestry, valued not as a casual snack, but as an indispensable "sour engine" in traditional culinary and medicinal practices.

The tree is small to medium-sized, typically reaching 5–10 meters in height, and is characterized by a short trunk that branches into a dense, attractive canopy of compound, feathery leaves. One of its most distinctive botanical features is its cauliflory, meaning the small, fragrant, yellowish-green to purplish flowers and the resulting fruits emerge directly from the trunk and main branches rather than the outer twigs. The fruit is an oblong, cylindrical berry, measuring 4–10 centimeters in length, with thin, smooth skin and five faint, longitudinal ridges. It transitions from a bright, waxy green when immature to a pale, yellowish-green when fully ripe. It thrives in well-draining, moist, sun-exposed soils but is notoriously sensitive to cold and strong winds.

The bilimbi is celebrated for its intensely sharp, acidic, and refreshing flavor, which is significantly more pungent and tart than even the strongest lemon or lime. Because of this extreme acidity, it is rarely eaten raw as a table fruit; instead, it serves as a foundational culinary souring agent. In Southeast Asian and Indian cuisines, it is crushed and used to make tangy curries, spicy sambals, and asam (sour) dishes. It is frequently preserved by brining in salt, drying in the sun, or pickling in vinegar to mellow its bite. In the Philippines, the raw fruit is often dipped in rock salt and served as an accompaniment to fish, while in other regions, it is processed into cooling drinks, concentrated syrups, and fermented into artisanal vinegars.

Nutritionally, the bilimbi is low in calories and provides a modest amount of dietary fiber, Vitamin C, and a surprising range of minerals including potassium and iron. Its biochemical profile is dominated by an exceptionally high concentration of oxalic acid, which gives the fruit its characteristic sourness and requires caution for individuals prone to kidney stones. Beyond its culinary utility, it is a powerhouse of specialized secondary metabolites, including tannins, flavonoids, and terpenoids. In traditional medicine across its native range, the fruit is widely used in folk remedies to treat coughs, skin inflammations, and as a natural cooling agent for fevers. Modern research into the plant has highlighted its potential as a source of anti-diabetic and antioxidant compounds, focusing on its ability to inhibit the activity of carbohydrate-digesting enzymes.