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