# What is the meaning of the equation of the change in entropy? [duplicate]

In my chemistry book, the formula for change in entropy is given as :

$$\int{dS} = \int{\frac{δq_{rev}}{T}}$$

What is the meaning of $$δq_{rev}$$? I know that it is the heat exchanged in a reversible process. But why $$δq_{rev}$$? Why not $$dq_{rev}$$? What is the difference between $$δq_{rev}$$ and $$dq_{rev}$$? What exactly is the meaning of $$δ$$ here and how is it different from $$d$$?

$$\delta Q$$ represents the inexact differential that means a change in heat from one state to another depends on the path. $$\Delta Q=\int_{\mathcal{P}} \delta Q \not= Q(\text{Final})-Q(\text{Initial})$$
It’s an inexact differential because, unlike properties like internal energy, pressure, temperature etc. heat (like work) is not a property so heat doesn’t “change” . The $$\delta$$ means amount of energy transferred in the form of heat. The amount transferred may then result in a change in the properties ($$dT$$, $$dp$$, $$dU$$, etc.) of the entities between which energy is transferred.