The areca nut (/ˈærᵻkə/ or /əˈriːkə/) is the seed of the areca palm (Areca catechu), which grows in much of the tropical Pacific (Melanesia and Micronesia), Southeast and South Asia, and parts of east Africa. This seed is commonly referred to as betel nut so it is easily confused with betel leaves that are often used to wrap it (paan). The term areca originated from a South Asian word during the 16th century, when Dutch and Portuguese sailors took the nut to Europe. Consumption has many harmful effects on health and is carcinogenic to humans. Various compounds present in the nut, most importantly arecoline (the primary psychoactive ingredient which is similar to nicotine), contribute to histologic changes in the oral mucosa. As with chewing tobacco, its use is discouraged by preventive efforts.

The areca nut is not a true nut, but rather a fruit categorized as a berry. It is commercially available in dried, cured and fresh forms. When the husk of the fresh fruit is green, the nut inside is soft enough to be cut with a typical knife. In the ripe fruit, the husk becomes yellow or orange and, as it dries, the fruit inside hardens to a wood-like consistency. At that stage, the areca nut can only be sliced using a special scissors-like cutter.

Usually for chewing, a few slices of the nut are wrapped in a betel leaf along with calcium hydroxide (slaked lime) and may include clove, cardamom, catechu (kattha) or other spices for extra flavouring. Betel leaf has a fresh, peppery taste, but it can also be bitter to varying degrees depending on the variety.

Areca nuts are chewed with betel leaf for their effects as a mild stimulant, causing a warming sensation in the body and slightly heightened alertness, although the effects vary from person to person.

The areca nut contains the tannins arecatannin and gallic acid; a fixed oil gum; a little terpineol; lignin; various saline substances; and three main alkaloids — arecoline, arecaidine and guvacine — all of which have vasoconstricting properties. The betel leaf chewed along with the nut contains eugenol, another vasoconstrictor. Tobacco leaf is often added to the mixture, thereby adding the effect of nicotine.

In parts of India, Sri Lanka and southern China, areca nuts are not only chewed along with betel leaf, but are also used in the preparation of Ayurvedic and traditional Chinese medicines. Powdered areca nut is used as a constituent in some dentifrices. Other traditional uses include the removal of tapeworms and other intestinal parasites by swallowing a few teaspoons of powdered areca nut, drunk as a decoction, or by taking tablets containing the extracted alkaloids. According to traditional Ayurvedic medicine, chewing areca nut and betel leaf is a good remedy against bad breath.[unreliable source?] Diplomat Edmund Roberts noted that Chinese people would mix areca nut with Uncaria gambir during his visit to China in the 1830s. After chewing a betelnut, the red residue from chewing the betelnut, buai pekpek, is generally spat out and is considered an eyesore. This has led many places to ban chewing buai as many people spit the pekpek on the ground rather than in a spit cup or waste receptacle.[citation needed]