Allium is the onion genus, with about 1250 species, making it one of
the largest plant genera in the world. They are perennial bulbous
plants that produce chemical compounds that give them a
characteristic onion or garlic taste and odor, and many are used as
food plants. Allium is classified in family Alliaceae although some
classifications have included it in the lily family (Liliaceae).
The flowers form an umbel at the top of a leafless stalk. The
bulbs vary in size between species, from very small (around 2–3 mm
in diameter) to rather big (8–10 cm). Some species Onion in the
general sense can be used for any plant in the genus Allium but used
without qualifiers usually means Allium cepa, also called the garden
onion. Onions (usually but not exclusively the bulbs) are edible
with a distinctive strong flavour and pungent odour which is
mellowed and sweetened by cooking. They generally have a papery
outer skin over a fleshy, layered inner core. Used worldwide for
culinary purposes, they come in a wide variety of forms and colours.
Onions may be grown from seed or very commonly from "sets". Onion
sets are produced by sowing seed very thickly one year, resulting in
stunted plants which produce very small bulbs. These bulbs are very
easy to set out and grow into mature bulbs the following year, but
they have the reputation of producing a less durable bulb than
onions grown directly from seed and thinned.