a plant, Allium cepa, of the amaryllis family, having an edible, succulent, pungent bulb.

any of certain similar plants.

the bulb of the onion plant.

the flavor or odor of this bulb.

Slang . a person

He's a tough onion.

containing or cooked with onions

onion soup.

of, pertaining to, or resembling an onion. ?

know one's onions

Slang . to know one's subject or business thoroughly; be capable or proficient.