Entropy
Defining entropy in an exact word or line is impossible. It can be viewed as a measure of molecular disorder or molecular randomness. As a system becomes more disordered, the positions of the molecules become less predictable and the entropy increases. Thus, the entropy of a substance is lowest in the solid phase and highest in the gas phase. Heat is, in essence, a form of disorganized energy, and some disorganization (entropy) will flow with heat. Work instead is an organized form of energy and is free of disorder or randomness, and thus free of entropy. There is no entropy transfer associated with energy transfer as work. Unlike energy, entropy is a non-conserved property.
According to Clausius inequality, 
is a cyclic integral of differential heat flow δQ at absolute temperature T. For a process change in entropy is defined by,
Entropy always increases. For all process 
To make it equality, add entropy generation term
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