Quasistatic process

In thermodynamics, a quasi-static process (also known as a quasi-equilibrium process. From the Latin "quasi", meaning ‘as if’), is a thermodynamic process that happens slowly enough for the system to remain in internal thermodynamic equilibrium. An example of this is quasi-static expansion, where the volume of a system changes so slowly that the pressure remains uniform throughout the system at each instant of time during the process. Such an idealized process is a succession of equilibrium states, characterized by infinite slowness.
Only in a quasi-static process can we exactly define intensive quantities (such as pressure, temperature, specific volume, specific entropy) of the system at every instant during the whole process; otherwise, since no internal equilibrium is established, different parts of the system would have different values of these quantities.
The theoretical term 'reversible process' is sometimes used. It refers to a theoretically convenient idealization that can in practice be realized only approximately, for exactly reversible processes are ruled out by the second law of thermodynamics. For example, slow compression of a system by a piston subject to friction is irreversible; although the system is always in internal thermal equilibrium, the friction ensures the generation of dissipative entropy, which goes against the definition of reversible. Any engineer would remember to include friction when calculating the dissipative entropy generation. Anx example of a slow process that is not even idealizable as reversible is slow heat transfer between two bodies on two finitely different temperatures, where the heat transfer rate is controlled by a poorly conductive partition between the two bodies— in this case, no matter how slowly the process takes place, the states of the composite system consisting of the two bodies is far from equilibrium, since thermal equilibrium for this composite system requires that the two bodies be at the same temperature.