- 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.
- a person: He's a tough onion.
- containing or cooked with onions: onion soup.
- of, relating to, or resembling an onion.
- to know one's subject or business thoroughly; be capable or proficient.
- an alliaceous plant, Allium cepa, having greenish-white flowers: cultivated for its rounded edible bulb
- the bulb of this plant, consisting of concentric layers of white succulent leaf bases with a pungent odour and taste
- any of several related plants similar to A. cepa, such as A. fistulosum (Welsh onion)
- to be fully acquainted with a subject
