The Collaborative International Dictionary of English v.0.48:

Onion \On"ion\, n. [F. ognon, fr. L. unio oneness, unity, a
   single large pearl, an onion. See One, Union.]
   1. (Bot.) A liliaceous plant of the genus Allium (Allium
      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.
      [1913 Webster]

   2. The flavor of an onion[1].
      [PJC]

   Onion fish (Zool.), the grenadier.

   Onion fly (Zool.) 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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Allium cepa
    n 1: bulbous plant having hollow leaves cultivated worldwide for
         its rounded edible bulb [syn: onion, onion plant,
         Allium cepa]