The entropy change d S of a system excluding its surroundings can be well-defined as a small portion of heat δ δ Q r e v transferred to the system during reversible process divided by the temperature T of the system during this heat transfer : d S = δ δ Q r e v T The reversible process is quasistatic (i.e., it occurs without any dissipation, deviating only infinitesimally from the thermodynamic equilibrium), and it may conserve total entropy. For example, in the Carnot cycle, while the heat flow from a hot reservoir to a cold reservoir represents the increase in the entropy in a cold reservoir, the work output, if reversibly and perfectly stored, represents the decrease in the entropy which could be used to operate the heat engine in reverse, returning to the initial state; thus the total entropy change may still be zero at all times if the entire process is reversible.