We overview the notion of entropy in thermodynamics. We start from the smooth case using differential forms on the manifold, which is the natural language for thermodynamics. Then the axiomatic definition of entropy as ordering on a set that is induced by adiabatic processes will be outlined. Finally, the viewpoint of category theory is provided, which reinterprets the ordering structure as a category of pre-ordered sets.

• 80-10 - Mathematical modeling or simulation for problems pertaining to classical thermodynamics
• 80A05 - Foundations of thermodynamics and heat transfer