[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

onion
     n 1: edible bulb of an onion plant
     2: bulbous plant having hollow leaves cultivated worldwide for
        its rounded edible bulb syn: onion plant, allium cepa
     3: an aromatic flavorful bulb
see also:
onion plant allium cepa 

[3] : 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."
   

Onion >> O Words
Onion, definition of term: Onion
onion_pag1.html

Onion Salt >> O Words
Onion Salt, definition of term: Onion Salt
onion+salt_pag1.html

Wild Onion >> W Words
Wild Onion, definition of term: Wild Onion
wild+onion_pag1.html

Green Onion >> G Words
Green Onion, definition of term: Green Onion
green+onion_pag1.html