The common name for the plant species Allium cepa is onion. It is also called the bulb onion or common onion and is the most widely cultivated species in the genus Allium.
Onions are biennial plants, although gardeners and farmers usually grow them as annuals. Their familiar bulb forms when the fleshy bases of the leaves swell underground and store food reserves. The plant is grown in many varieties, including yellow, red, and white onions.
Allium cepa is related to garlic, leek, scallion, and chives, which explains why those names are sometimes confused in gardening questions. However, they are not the same species: garlic is Allium sativum, while leek is Allium ampeloprasum. Carl Linnaeus formally described Allium cepa in 1753. When chopped, onions release irritating compounds that can make the eyes water.