phonon (fōˈnŏn) [key], quantum of vibrational energy. The atoms of any crystal are in a state of vibration, their average kinetic energy being measured by the absolute temperature of the crystal. In certain phenomena it becomes evident that this energy is divided into discrete bundles (see quantum theory); the energy bundles behave like particles in some respects and are termed phonons. These effects are most apparent at low temperatures where only a few phonons are present. For example, interactions between phonons and electrons are thought to be responsible for such phenomena as superconductivity.