The word quantum (plural: quanta) comes from the Latin "quantus," for "how much."
In physics, a quantum (plural: quanta) is an indivisible entity of energy. A photon, for instance, being a unit of light, is a "light quantum." In combinations like "quantum mechanics", "quantum optics", etc., it distinguishes a more specialized field of study.
Behind this, one finds the fundamental notion that a physical property may be "quantized", referred to as "quantization". This means that the magnitude can take on only certain discrete numerical values, rather than any value, at least within a range. For example, the energy of an electron bound to an atom (at rest) is quantized. This accounts for the stability of atoms, and matter in general.
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