Onions, or Alium cepa L, are vegetables of the genus Allium There are many varieties, ranging in pungency, shape and color (red, yellow and white). Mature onion bulbs are the most commonly eaten, although onions can be consumed at any stage. Onions are available fresh, frozen, pickled, dried and powdered. Onions contain low amounts of essential nutrients.
Onions are toxic to many animals, like dogs and cats (ref).