The onion is an
edible bulb. While it is a
vegetable at heart, it also acts as a spice inasmuch as it can provide an aromatic undertone to various meat and vegetable dishes, without being a major ingredient. The characteristic appearance of the onion is well known, but there are many variations of colour, shape and size.The colour varies from
white to red to purple, the shape from spherical to almost conical, and the diameter at the largest point from 10mm (1/2in) to 8cm (3in) or 'more.
Onions should be firm, though not rock hard. The papery skin should be tight over the surface of the bulb. Spring onions, or scallions, are immature plants where the bulb has not completely formed. They may be cylindrical, the green stem shading into the white bulblet, which may be almost spherical. Onions are also available in processed form, as dried flakes and powder, or liquid.