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Physics (specifically, quantum mechanics) uses the term quantum (plural: quanta) for a minimal amount of something: it is a minimum amount that can result in a response. An example quantum is the minimal amount of electromagnetic radiation's (EMR's) energy at a given frequency. A quantity subject to such minimal amounts is said to be quantized. The word is clearly related to quantity, and had previously been used as a synonym of that word.
Such a minimum amount of EMR can be treated as a particle (termed a photon), and a beam of EMR acts like a stream of such photons. However, some situations famously show the behavior of a single one of these "particles" to be affected by both of a pair of side-by-side holes in a barrier that it is passing through, a situation where considering EMR to be a wave spanning both holes offers a more straightforward description.
The term, quantum is also often used to indicate "having to do with quantum methods", e.g., in the phrase, quantum computing, referring to the use of quantum mechanical phenomena to help carry out calculations.