First law of thermodynamics: The first law of thermodynamics states that the change in the internal energy of a system is equal to the sum of the work done on the system and the heat added to the system.
Thermodynamic system: A thermodynamic system is a defined region of space that is under consideration for a thermodynamic analysis, separated from its surroundings by a boundary.
Reversible process: A reversible process is a thermodynamic process that can be reversed without leaving any trace on the system or its surroundings.