On″ion (?), n. [F. ognon, fr. L. unio oneness, unity, a single large pearl, an onion. See One, Union.] (Bot.) A liliaceous plant of the genus Allium (A. cepa), having a strong-flavored bulb and long hollow leaves; also, its bulbous root, much used as an article of food. The name is often extended to other species of the genus.
Onion fish(Zoöl.), the grenadier. — Onion fly(Zoöl.) a dipterous insect whose larva feeds upon the onion; especially, Anthomyia ceparum and Ortalis flexa. — Welsh onion. (Bot.) See Cibol. — Wild onion(Bot.), a name given to several species of the genus Allium.