A phonon is the quantum of a vibration of a crystal lattice.
In a solid, atoms are linked by forces and can vibrate around their equilibrium positions. Quantum mechanics allows these collective vibrational modes to possess discrete energies. Each unit of vibrational excitation is called a phonon, analogous in some ways to the photon as a quantum of electromagnetic excitation.
Phonons are quasiparticles rather than independent elementary particles. They describe organized behavior involving many atoms and are especially useful for understanding heat capacity, thermal conductivity, sound propagation, and interactions in solids. Their properties depend on the crystal structure and the mode of vibration.
A phonon is not the same as a photon: photons are excitations of the electromagnetic field, whereas phonons arise from collective motion in matter. Magnons are another type of quasiparticle, representing collective spin-wave excitations in magnetic materials.