[noun] The mathematical model that describes the behavior of subatomic particles, beyond classical particle models, by incorporating the quantization of energy and wave-particle duality. A quantum is a fixed amount of energy absorbed or released during an exchange of radiation. Quantum exchanges are often associated with the transition of particles such as electrons between an excited and ground state.

[noun] A set of four numbers that are commonly used to describe the position of an electron within an atom. These include: n, the principal quantum number; l, the angular or azimuthal quantum number; m, the magnetic quantum number; and s, the electron spin quantum number.