In some cases, it is important to distinguish between reversible and quasistatic processes.

In the limit δV to zero, this becomes an ideal quasistatic process, albeit an irreversible one.

A quasistatic process is an idealized model of a thermodynamic process that happens infinitely slowly.

The quantum mechanical definition is closer to the thermodynamical concept of a quasistatic process, and has no direct relation with heat exchange.

Thermodynamic cycles are often represented mathematically as quasistatic processes to model the workings of an actual device.

Cycles composed entirely of quasistatic processes can operate as power or heat pump cycles by controlling the process direction.

A central notion for this connection is that of quasistatic processes, namely idealized, "infinitely slow" processes.

The original definition of Carathéodory was limited to reversible, quasistatic process, described by a curve in the manifold of equilibrium states of the system under consideration.

However, some quasistatic processes are irreversible, if there is heat flowing (in or out of the system) or if entropy is being created in some other way.

A quasistatic process ensures that the system will go through a sequence of states that are infinitesimally close to equilibrium (so the system remains in quasistatic equilibrium), in which case the process is typically reversible.

An example of a quasistatic process that is not reversible is a compression against a system with a piston subject to friction - Although the system is always in thermal equilibrium, the friction ensures the generation of dissipative entropy, which directly goes against the definition of reversible.