A
quantum system is a portion of the whole
Universe (environment or physical world) which is taken under consideration to make analysis or to study for
quantum mechanics pertaining to the
wave-particle duality in that system and everything else than this system i.e. environment is studied only to observe its effects on the system. A quantum system involves the wave function and its constituents, such as the
momentum and
wavelength of the wave for which wave function is being defined.