In physics, a plasmon is a quantum of plasma oscillation. The plasmon can be considered aquasiparticle since it arises from the quantization of plasma oscillations, just like phonons are quantizations of mechanical vibrations. Thus, plasmons are collective oscillations of the free electron gas density, for example, at optical frequencies. Plasmons can couple with a photon to create another quasiparticle called a plasma polariton.
Since plasmons are the quantization of classical plasma oscillations, most of their properties can be derived directly from Maxwell's equations.