For a spontaneous process,
 

 For a spontaneous process in an isolated system, the change in entropy is positive, i.e, $\Delta S>O$.

However, if a system is not isolated, the entropy change of both the system and surroundings are to be taken into account because system and surroundings together constitute the isolated system thus, the total entropy change $\left(\Delta S_{\text{total }}\right)$is sum of the change in entropy of the system $\left(\Delta S_{\text{system }}\right)$ and the change in entropy of the surroundings $\left(\Delta S_{\text{purroundings }}\right)$
i.e., $\Delta S_{\text{total }}=\Delta S_{\text{system }}+\Delta S_{\text{Surrounding }}$ for a spontenceus process, $\Delta S_{\text{total }}$ must be positive, i.e., $\Delta S_{\text{total }}$ is also termed as $\Delta S_{\text{universe" }}$