Step 1: Understanding the Concept:
In thermodynamics, reversibility implies that a system can undergo a process and then perfectly return to its initial state without increasing the entropy of the universe.
For a process to be practically perfectly reversible, there must be no dissipating forces (like friction), and the system must remain in thermodynamic equilibrium throughout the transformation.
Step 2: Detailed Explanation:
To maintain thermodynamic equilibrium continuously, any change made to the system must happen at an infinitesimally slow rate.
This ensures that the intensive properties (like temperature and pressure) are uniform throughout the system at any given moment.
A process carried out under these extremely slow conditions is explicitly defined as a "quasi-static" process (quasi = almost, static = stationary).
While isothermal or adiabatic processes can be reversible under ideal conditions, the fundamental procedural requirement for ANY process to be reversible is that it must be quasi-static.
Step 3: Final Answer:
The process that must occur infinitesimally slowly to remain reversible is the quasi-static process.