what is the usual definition of entropy ? What is the unit of entropy ?

Explanation

(i) Entropy is a thermodynamic state function that is a measure of the randomness or disorderliness of the system .
(ii) For a reversible change taking place at a constant temperature (T), the change in entropy (`DeltaS`) of the system is equal to heat energy absorbed or evolved (q) by the system divided by the constant temperature (T)
`DeltaS_"sys"=q_"rev"/T`
(iii) SI unit of entropy is `JK^(-1)`