[1] : 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 onion1.
      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
see also:
one union allium allium cepa onion fish onion fly 
anthomyia ceparum ortalis flexa welsh onion cibol wild onion 

[2] : WordNet (r) 2.0

allium cepa
     n : bulbous plant having hollow leaves cultivated worldwide for
         its rounded edible bulb syn: onion, onion plant
see also:
onion onion plant 

Allium Cepa >> A Words
Allium Cepa, definition of term: Allium Cepa
allium+cepa_pag1.html

Allium >> A Words
Allium, definition of term: Allium
allium_pag1.html