onion

American  

[uhn-yuhn] / ˈʌn yən /

noun

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

2. any of certain similar plants.

3. the bulb of the onion plant.

4. the flavor or odor of this bulb.

5. Slang. a person.

   He's a tough onion.

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adjective

1. containing or cooked with onions.

   onion soup.

2. of, relating to, or resembling an onion.

idioms

1. know one's onions, to know one's subject or business thoroughly; be capable or proficient.

onion British  

/ ˈʌnjən /

noun

1. an alliaceous plant, Allium cepa, having greenish-white flowers: cultivated for its rounded edible bulb

2. the bulb of this plant, consisting of concentric layers of white succulent leaf bases with a pungent odour and taste

3. any of several related plants similar to A. cepa, such as A. fistulosum (Welsh onion)

4. slang to be fully acquainted with a subject

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onion More Idioms  

1. see know one's stuff (onions).

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Other Word Forms

• onionlike
  adjective
• oniony
  adjective

Etymology

Origin of onion

1325–75; Middle English onyon < Old French oignon < Latin ūniōn- (stem of ūniō ) a unity, large pearl, onion; see union

Explanation

An onion is a plant that grows as a bulb underground and can be harvested and eaten. Note: onions may be tasty, but they sure can make your breath stink. Onions are bulbs, like tulips or daffodils — most of the plant is hidden in the soil, with roots stretching down and stalks that grow upward and eventually bloom with small flowers. Onions are typically harvested in the fall, and they come in many shapes, sizes, and colors, all of which can be eaten. The Latin root of onion, unionem, was used in colloquial Roman to mean both "a type of onion" and "a pearl."

Example Sentences

Then I’m getting the lime roll, the albacore crispy onion, the garlic sashimi, and I’m going to keep ordering and ordering and be so happy.

From Los Angeles Times • May 22, 2026

Add the onion and cook until softened and lightly golden, about 4 minutes.

From Salon • May 19, 2026

Spooned into warm tortillas and topped with cilantro, green onion and another shake of paprika, these lentils feel far more luxurious than their ingredient list would suggest.

From Salon • May 19, 2026

A Ramp Gibson, a riff on the martini, came with a pickled ramp garnish instead of a pickled onion.

From The Wall Street Journal • May 13, 2026

Anya grinned to herself as she reached for another onion stalk.

From Anya and the Dragon by Sofiya Pasternack