1. The branch of physicochemical science concerned with heat and energy and their conversions one into the other involving mechanical work.
2. The branch science which is concerned with the study of energy and with the relationship of heat transfer and work to other forms of energy.
3. The study of the flow of heat.
4. A branch of science dealing with heat, energy, and the interconversion of these, and with related problems.

Thermodynamics is the physical science that accounts for the transformation of thermal energy into mechanical energy and its equivalent forms in which temperature is a significant factor; such as, electricity and self-organization of complex systems.