Explain Third law of thermodynamics.

Third law of thermodynamics states that at absolute zero temperature, the entropy of system approaches a constant value. This value depends on some parameters like pressure and magnetic field. The value of minimum possible energy of the system at absolute zero is zero. Temperature is defined as

$T={\left(\frac{\partial U}{ds}\right)}_{V,N}$

According to third law $\underset{T\to 0}{\lim }\Delta S=0$