The branch of physics governing energy transfer as heat and work, and the evolution of entropy.
zeroth law: thermal equilibrium defines temperature
first law: energy is conserved — heat in equals work out plus internal energy change
second law: entropy of an isolated system never decreases
third law: entropy approaches zero as temperature approaches absolute zero
free energy determines spontaneous processes and equilibrium
connects mechanics at the microscopic scale to macroscopic observables
underpins chemistry, biology, and information theory