Entropy Change and Clausius Inequality

$$\Delta S = \frac{q_{\mathrm{rev}}}{T}$$

(for a reversible process at constant temperature)

Unit: $$[\Delta S] = \mathrm{J\,K^{-1}}$$

Clausius inequality for any cyclic process:
$$\oint \frac{\delta q}{T} \le 0$$
(equality for reversible cycles, < for irreversible cycles)