A phonon is a quantized mode of vibration occurring in a rigid crystal lattice, which can be thought of as a particle of sound. It represents the collective excitations of atoms in a solid material, allowing energy to be transferred through the lattice. Phonons play a crucial role in various physical properties of materials, such as thermal conductivity and electrical conductivity. In solid-state physics, phonons are essential for understanding phenomena like superconductivity and thermal expansion. They can be classified into two main types: acoustic phonons, which are related to sound waves, and optical phonons, which involve the relative motion of atoms in the lattice.