phonon The quantized unit of lattice vibrations in a solid. The elastic waves in solid are of two kinds, longitudinal and transverse. These waves, called the lattice vibrations, have energies quantized in unit of h. This is similar to photons of electromagnetic radiations. Phonons are characterized by their energy, momentum, wavelength and wave vector. The concept of phonons gives insight into various physical processes in solids such as thermal and electrical conduction, elastic wave propagation, neutron scattering in solids, superconductivity, etc.