The notion of a “quasiparticle” is one of the fundamental concepts in the theory of condensed substances (and, particularly, solid-state physics). The introduction of this concept has significantly simplified the methods for describing the wide range of processes in many particle systems with strong interaction. The concept of quasiparticles (whose development was largely contributed by L.D. Landau) enabled the reduction of the complex dynamics of systems comprised of strongly interacting particles to a simpler dynamics of quasi-independent objects.

The quasiparticles and the usual elementary particles manifest both similarities and key differences. Like ordinary particles, quasiparticles can be localisised in space and maintain their localisisation while in motion; they can interact both with particles and other quasi-particles (in this case the mechanical laws of conservation of quasimomentum and quasienergy remain valid; the concept of effective mass is applied to quasiparticles with the quadratic dispersion law, , i.e. the behaviour of such quasiparticles is very similar to the behaviour of ordinary particles). On the other hand, quasiparticles (as opposed to ordinary particles) cannot exist outside of a medium, whose elementary excitations are essential for their existence.

By their internal structure all quasiparticles can be divided into two classes: single-particle excitations, i.e. single particles covered with "fur" due to interaction with other excitations (e.g., a hole surrounded by particle-hole pairs) and collective excitations, i.e. sets of equal components (e.g. exciton, plasmon, etc.).

Examples of quasiparticles include a phonon, plasmon, exciton, hole, and magnon. There also exist more complicated quasiparticles representing combinations of the above. If two types of quasiparticles in one system have similar values of energy and momentum, then these quasiparticles mix (hybridize) with the formation of two new quasiparticles, each of which has some characteristics of both initial quasiparticles. Thus, mixing a photon with an exciton or an optical phonon results in the formation of polaritons; mixing a photon with a magnon results in the formation of photomagnons.