quantum

American  

[kwon-tuhm] / ˈkwɒn təm /

noun

quanta plural

1. quantity or amount.

   the least quantum of evidence.

2. a particular amount.

3. a share or portion.

4. a large quantity; bulk.

5. Physics.

   1. the smallest quantity of radiant energy, equal to Planck's constant times the frequency of the associated radiation.

   2. the fundamental unit of a quantized physical magnitude, as angular momentum.

---

adjective

1. sudden and significant.

   a quantum increase in productivity.

quantum British  

/ ˈkwɒntəm /

noun

1. physics

   1. the smallest quantity of some physical property, such as energy, that a system can possess according to the quantum theory

   2. a particle with such a unit of energy

2. amount or quantity, esp a specific amount

3. (often used with a negative) the least possible amount that can suffice

   there is not a quantum of evidence for your accusation

4. something that can be quantified or measured

5. (modifier) loosely, sudden, spectacular, or vitally important

   a quantum improvement

---

quantum Scientific  

/ kwŏn′təm /

quanta plural

1. A discrete, indivisible manifestation of a physical property, such as a force or angular momentum. Some quanta take the form of elementary particles; for example, the quantum of electromagnetic radiation is the photon, while the quanta of the weak force are the W and Z particles.

2. See also quantum state

---

Other Word Forms

Noun Inflected Forms

• quanta
  plural

Etymology

Origin of quantum

1610–20; noun use of neuter of Latin quantus how much

Explanation

While quantum refers to a general quantity or size, it is most often used in physics as a measure of the smallest amount of something — usually energy — that something can possess. The plural form is quanta. In the 17th century, the word quantum, from the Latin word for "how much," referred to a portion. Quantum is usually a noun referring to a specific amount of something. However, it can also be used as an adjective, as in the phrase "quantum leap," where it refers to a sudden, important change. Outside of physics, the word might be used with "not" to refer to something insignificant, as in "There's not a quantum of truth in what you say."

Example Sentences

Quantum Systems is seeking to expand production and ramp up investments in software and AI capabilities.

From The Wall Street Journal • Jul. 2, 2026

Quantum Systems makes multisensor unmanned aerial systems that can assist government agencies and commercial clients with aerial data intelligence.

From The Wall Street Journal • Jul. 2, 2026

Shanghai-based Taiyi Quantum announced last week that it had completed a 300 million yuan, or roughly $44 million, Pre-A funding round, nearly double the original fundraise target of $29 million.

From Barron's • Jun. 29, 2026

Xanadu Quantum Technologies CEO Christian Weedbrook described China’s quantum effort as “a huge black box” in a Barron’s interview earlier this year.

From Barron's • Jun. 29, 2026

Quantum mechanics, on the other hand, describes a jerky and discontinuous universe.

From "Zero: The Biography of a Dangerous Idea" by Charles Seife