onion

American  

[uhn-yuhn] / ˈʌn jən /

noun

onions plural

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.

---

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

---

onion More Idioms  

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

---

Other Word Forms

Derived Forms

• onionlike
  adjective
• oniony
  adjective

Inflected Forms

Nouns

• onions
  plural

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

The first two episodes of Season 3 are out, and this week’s was particularly satisfying, Slate’s Jenny G. Zhang and Rebecca Onion write.

From Slate • Jun. 30, 2026

It’s The Onion made flesh, the kind of absurdity that would cause any 2014 time-traveler to instantly smell burnt toast.

From Slate • Jun. 14, 2026

Other cheese options include freshly grated gruyère, a classic choice for French Onion soup; Parmigiano-Reggiano, which is perfect in tomato-forward pasta soups; and crumbly goat cheese, which complements butternut squash soup or pumpkin bisque.

From Salon • Jun. 13, 2026

He’s one of the founding writers of the Onion, the satirical website.

From Los Angeles Times • Jun. 11, 2026

I cannot share secrets—you know how the Pickled Onion is about information slipping into the wrong hands.

From "The Detective's Assistant" by Kate Hannigan