A branch of physics that studies the processes involved in the reversible transformation of heat into mechanical work, of mechanical work into heat, or the flow of heat from a hotter to a colder body. It does so on a more macroscopical level than statistical mechanics. The first and second laws of thermodynamics aid in understanding processes like these. There are two other laws: the third law, or the Nernst heat theorem (as a homogeneous system approaches a temperature of absolute zero, then its entropy tends to zero), and the zeroth law (if two bodies are each separately in thermal equilibrium with a third body, then all three bodies are in thermal equilibrium with each other).
See Bridge law (or principle), Entropy, First law of thermodynamics, Irreversible thermodynamics, Law, Open system, Second law of thermodynamics, Statistical mechanics