Why is onion called onion?

Answer 1

Onion is a term used for many plants in the genus allium. They are known by the common name "onion" but, used without qualifiers, it usually refers to Allium cepa. Allium cepa is also known as the garden onion or bulb onion. Above ground, the onion shows only a single vertical shoot; the bulb grows underground, and is used for energy storage, leading to the possibility of confusion with a tuber, which it is not. It is a close relative to garlic.

Answer 2

It is a vegetable, can be red or white.

The word onion came into the English language from French 'union', from 'oignon'/'oingnon', in the twelfth century. The earliest records of the word we have puts it in ancient Roman usage 'unionem/unio' with the literal meaning of 'pearl', or 'unity' ('one').

Today the word refers to any of the family allium, grown worldwide for culinary and medicinal uses, and still occurring in the wild in some areas. The plant best known is the Allium cepa, which is the bulb we are probably most familiar with, the brown-skinned onion.

See the link below for information on types and uses of onions.