A quasi-equilibrium refers to a quasi-equilibrium process in thermodynamics, which is a process in which the system only deviates from equilibrium by infinitesimal amounts. Quasi-equilibrium processes can be used to closely approximate real processes.
All of the intermediate states during quasi-equilibrium processes can be treated as equilibrium states, which simplifies the mathematical treatment. Most quasi-equilibrium processes involve very slow changes, such as the slow compression of a piston. Quasi-equilibrium is an important concept for thermodynamics, because it is impossible for any real system to remain in true equilibrium while a process occurs. Additionally, non-equilibrium states cannot be dealt with using thermodynamic equations, so the approximation of quasi-equilibrium allows thermodynamic processes to be understood.