A phonon is the vibration quantum of a crystalline lattice. Even at absolute zero (0K), the basis atoms of a crystal vibrate about their average equilibrium positions. The concept of a crystal having a precisely-known, static structure is thus only an approximation. Solid-state physicists find it very useful to make this approximation, known as the Born-Oppenheimer Approximation or adiabatic approximation, when calculating bandstructure--the electron eigenstates and eigenenergies in a solid. The treatment of crystalline vibrations as quantized packets (phonons) allowed physicists to explain why the specific heat of solids is proportional to T3 at very low temperatures.

Phonons are important for less academic reasons. The vibrations of a crystal act to scatter electrons (and holes). In a silicon MOSFET (a type of transistor), phonons are the fundamental barrier to current flow. The speed of a digital integrated circuit such as a microprocessor is dependent on how fast MOSFETs can charge and discharge capacitors. Since phonons limit current, they greatly degrade circuit speed. At lower temperatures, crystals vibrate less. For this reason, microprocessors could run many times faster if cooled (by liquid nitrogen for instance).