Density of states (DOS) is a property in condensed matter physics that quantifies how closely packed energy levels are in some material. It is often expressed as a function N(E) of the internal energy E. It is usually used with electronic energy levels in a solid.

A more precise definition is as follows: <math>N(E)\,dE<math> is the number of allowed energy levels per unit volume of the material, within the energy range <math>E<math> to <math>E + dE<math>.