For a spontaneous process in an isolated system ∆S should be positive i.e ∆S > 0.
In thermodynamics, S stands for entropy. The entropy of any system is defined as the degree of randomness in it.
A spontaneous process is an irreversible process, and some external agents could only reserve it.
For a spontaneous process, the total entropy change, ΔStotal is always greater than zero.
The total entropy of the system can be given by the following equation
ΔStotal=ΔSsys + ΔSsurr
ΔStotal= total entropy change for the process
ΔSsys = entropy change of the system
ΔSsurr = entropy change of the surrounding