Definitions for "Phonon"
A quantum of acoustic energy, analogous to the quantum of electromagnetic radiation, the photon. Thermal excitations in a crystal or in an elastic continuum can be described as a population of phonons (analogous to blackbody electromagnetic radiation). In highly inhomogeneous solids, a description in terms of phonons breaks down and localized vibrational modes become important.
Quantum of vibrational energy.
Quantum of acoustic energy, the level of which is a function of the frequency of the acoustic wave.