Onions are bulbs.
The bulb is formed of hollow leaves, thickened into 1.5 to 5mm fleshy layers which overlap. Bulb skins are yellow, white or red and range in size from 6 to 14cm in diameter. Onions have become overwhelming, not only because there are so many kinds, but also because the categories are arbitrary. This would probably serve most purposes as well as any other, and it provides a practical method of distinguishing a scallion from a baseball shaped yellow globe. UK grows about million kilograms of onions.