Allium is a monocot genus of flowering plants, informally referred to as the onion genus. The generic name Allium is the Latin word for garlic. The genus, including the various edible onions, garlics, chives, and leeks, has played a pivotal role in cooking worldwide, as the various parts of the plants, either raw or cooked in many ways, produce a large variety of flavors and textures.

The genus contains hundreds of distinct species; many have been harvested through human history, but only about a dozen are still economically important today as crops or garden vegetables. Many others are cultivated as ornamental plants.

Allium is taxonomically difficult and species boundaries are unclear. Most authorities accept about 750 species. Estimates of the number of species have been as low as 260, and as high as 860. The type species for the genus is Allium sativum.

Allium species occur in temperate climates of the Northern Hemisphere, except for a few species occurring in Chile (such as A. juncifolium), Brazil (A. sellovianum) or tropical Africa (A. spathaceum). They can vary in height between 5 cm and 150 cm. The flowers form an umbel at the top of a leafless stalk. The bulbs vary in size between species, from very small (around 2–3 mm in diameter) to rather large (8–10 cm). Some species (such as Welsh onion, A. fistulosum) develop thickened leaf-bases rather than forming bulbs as such.Allium is a genus of perennial bulbous plants that produce chemical compounds (mostly cysteine sulfoxide) that give them a characteristic onion or garlic taste and odor. Many are used as food plants, though not all members of the genus are equally flavorful. In most cases, both bulb and leaves are edible. Their taste may be strong or weak, depending on the species and on ground sulphur (usually as sulfate) content (in the rare occurrence of sulphur-free growth conditions, all Allium species will lack their usual pungency altogether).

In the APG III classification system, Allium is placed in the family Amaryllidaceae, subfamily Allioideae (formerly the family Alliaceae). In some of the older classification systems, Allium was placed in Liliaceae. Molecular phylogenetic studies have shown this circumscription of Liliaceae is not monophyletic.

Allium is one of about 57 genera of flowering plants with more than 500 species. It is by far the largest genus in the Amaryllidaceae, and also in the Alliaceae in classification systems in which that family is recognized as separate.