The leek is a vegetable that belongs, along with onion and garlic, to the genus Allium, currently placed in family Amaryllidaceae, subfamily Allioideae. Historically, many scientific names were used for leeks, which are now treated as cultivars of Allium ampeloprasum. Two related vegetables, elephant garlic and Kurrat, are also cultivars of A. ampeloprasum, although different in their uses as food.

The edible part of the leek plant is a bundle of leaf sheaths that is sometimes erroneously called a stem or stalk.

Rather than forming a tight bulb like the onion, the leek produces a long cylinder of bundled leaf sheaths that are generally blanched by pushing soil around them (trenching). They are often sold as small seedlings in flats that are started off early in greenhouses, to be planted out as weather permits. Once established in the garden, leeks are hardy; many varieties can be left in the ground during the winter to be harvested as needed.