Condensed matter comprises physical systems such as solids and liquids in which interactions among many constituents determine the observed properties. The term refers to the material system; condensed matter physics studies its structure, dynamics and phases.
A crystal illustrates the change of viewpoint. Its chemical atoms can have collective vibrational modes described as phonons, while its electronic response can involve energy bands rather than isolated-atom energy levels. Collective excitations and phase transitions show why a many-constituent system can display useful organizing principles that are not apparent from one atom alone.
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