The Betel (Piper betle) is a spice whose leaves have medicinal properties. The plant is evergreen and perennial, with glossy heart-shaped leaves and white catkins, and grows to a height of about 1 metre. The Betel plant originated in Malaysia and now grows in India and Indonesia.

The active ingredients of betel oil, which is obtained from the leaves, are betel-phenol (or chavibetol or 3-hydroxy-4-methoxyallylbenzene, which gives a smoky aroma), chavicol and cadinene .

In India the leaves are chewed together with mineral lime (calcium oxide) and the areca nut which, by association, is sometimes inaccurately called the "betel nut". The lime acts to keep the active ingredient in its freebase or alkaline form, thus enabling it to enter the bloodstream via sublingual absorption. The areca nut contains the alkaloid arecoline , which promotes salivation (the saliva is stained red). This combination, known as a "betel quid", has been used for several thousand years. Tobacco is sometimes added.

Betel leaves are used as a stimulant, an antiseptic and a breath-freshener. In Ayurvedic medicine, they are used as an aphrodisiac. In Malaysia they are used to treat headaches, arthritis and joint pain. In Thailand and China they are used to relieve toothache. In Indonesia they are drunk as an infusion and used as an antibiotic. They are also used in an infusion to cure indigestion, as an ointment or inhalant to cure headache, as a topical cure for constipation, as a decongestant and as an aid to lactation.

A related plant P. sarmentosum, which is used in cooking, is sometimes called "wild betel leaf".