reversibility

Capable of assuming or producing either of two states; of or relating to a process, such as a chemical reaction or a phase change, in which the system undergoing the process can be returned to its original state. [answers.com]
From a good article that I found, but have not yet finished reading, Reversibility and Entropy, “The defining characteristic of a reversible process is that it can (or could) proceed equally well either in the forward or the reverse temporal directions.”