Source : Webster's Revised Unabridged Dictionary (1913)

Onion \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} (Zo["o]l.), the grenadier.

   {Onion fly} (Zo["o]l.) 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}.

Source : WordNet®

wild onion
     n : any of various plants of the genus Allium with edible bulbs
         found growing wild