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):

onion
    n 1: the bulb of an onion plant
    2: bulbous plant having hollow leaves cultivated worldwide for
       its rounded edible bulb [syn: onion, onion plant, Allium
       cepa]
    3: an aromatic flavorful vegetable

Easton's 1897 Bible Dictionary:

Onion
   The Israelites in the wilderness longed for the "onions and
   garlick of Egypt" (Num. 11:5). This was the _betsel_ of the
   Hebrews, the Allium cepe of botanists, of which it is said that
   there are some thirty or forty species now growing in Palestine.
   The onion is "the 'undivided' leek, _unio_, _unus_, one."