In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids. Often referred to as a quasiparticle, it represents an excited state in the quantum mechanical quantization of the modes of vibrations of elastic structures of interacting particles.

Phonons play a major role in many of the physical properties of condensed matter, such as thermal conductivity and electrical conductivity. The study of phonons is an important part of condensed matter physics.

The concept of phonons was introduced in 1932 by Russian physicist Igor Tamm. The name phonon comes from the Greek word φωνή (phonē), which translates as sound or voice because long-wavelength phonons give rise to sound. Wikipedia, Phonon

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Sound, in the day-to-day classical world, can be understood as a density wave in a medium. However, from the perspective of quantum mechanics, sound can also be described as a particle: the phonon. This particle, the sound quantum, represents the smallest amount of energy that occurs as an acoustic wave at a certain frequency. In order to see and study a single quanta of sound, the number of phonons must be minimized. https://phys.org/news/2024-01-quantum-ground-state-waveguides-scientists.html