What defines a reversible cell in electrochemistry?

A reversible cell in electrochemistry is defined by its ability to allow the electrochemical reactions to be reversed by applying an external voltage. This characteristic is fundamental because it means that the cell can both generate electricity through spontaneous reactions and store energy by driving those reactions in the opposite direction when needed. This reversibility is essential for applications like rechargeable batteries, where the same electrochemical system can be used multiple times for discharging and recharging.

In contrast, the other options either describe cells with limitations or do not align with the concept of reversibility. For instance, cells that cannot reverse their reactions would be classified as irreversible, while the notion that a cell must operate at high temperatures is not a defining aspect of reversibility. Moreover, a cell that generates electricity without any external input describes a self-sustaining process, which isn't specifically related to the concept of reversibility in electrochemical processes.