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.”

Definitions of quantum
  1. noun

    (physics) the smallest discrete quantity of some physical property that a system can possess (according to quantum theory)

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    a quantum of energy (in a crystal lattice or other system) that has position and momentum and can in some respects be regarded as a particle
    type of:

    amount, measure, quantity

    how much there is or how many there are of something that you can quantify

  2. noun

    a discrete amount of something that is analogous to the quantities in quantum theory

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    type of:


    the concept that something has a magnitude and can be represented in mathematical expressions by a constant or a variable

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