A quasi-static process is a thermodynamic process that happens slowly enough for the system to remain in internal physical thermodynamic equilibrium/Book%3A_University_Physics_II_-Thermodynamics_Electricity_and_Magnetism(OpenStax)/03%3A_The_First_Law_of_Thermodynamics/3.05%3A_Thermodynamic_Processes). In other words, the change in state is made infinitesimally slowly so that at each instant, the system can be assumed to be at a thermodynamic equilibrium with itself and with the environment/Book%3A_University_Physics_II_-Thermodynamics_Electricity_and_Magnetism(OpenStax)/03%3A_The_First_Law_of_Thermodynamics/3.05%3A_Thermodynamic_Processes). The system needs to maintain equilibrium with the surroundings at all times during the process in order for the process to be classified as a quasi-static process. Quasi-static processes are done slowly enough that the system remains at thermodynamic equilibrium at each instant, despite the fact that the system changes over time/Book%3A_University_Physics_II_-Thermodynamics_Electricity_and_Magnetism(OpenStax)/03%3A_The_First_Law_of_Thermodynamics/3.05%3A_Thermodynamic_Processes). The thermodynamic equilibrium of the system is necessary for the system to have well-defined values of macroscopic properties such as the temperature and the pressure of the system at each instant of the process/Book%3A_University_Physics_II_-Thermodynamics_Electricity_and_Magnetism(OpenStax)/03%3A_The_First_Law_of_Thermodynamics/3.05%3A_Thermodynamic_Processes). Since quasi-static processes cannot be completely realized for any finite change of the system, all processes in nature are non-quasi-static/Book%3A_University_Physics_II_-Thermodynamics_Electricity_and_Magnetism(OpenStax)/03%3A_The_First_Law_of_Thermodynamics/3.05%3A_Thermodynamic_Processes).