In physics, a quantum (plural: quanta) is the minimum amount of any physical entity involved in an interaction. Behind this, one finds the fundamental notion that a physical property may be "quantized," referred to as "the hypothesis of quantization". This means that the magnitude of the physical property can take on only certain discrete values.
For example, a photon is a single quantum of (visible) light as well as a single quantum of all other forms of electromagnetic radiation, and can be referred to as a "light quantum". The energy of an electron bound to an atom is also quantized, and thus can only exist in certain discrete values. As a result, atoms are stable, and hence matter in general is stable.
As incorporated into the theory of quantum mechanics, this quantization of the energy of electrons and the resulting implications are regarded by physicists as part of the fundamental framework for understanding and describing nature.