Before anything else is discussed about thermodynamics, it is important to define what a thermodynamic system and process is.

An important concept in thermodynamics is the system. A system is the region being studied. A system is separated from the remainder of the universe by a boundary which may be imaginary or not, but which limits the volume of region being studied. The possible exchanges of work, heat, or matter between the system and the surroundings take place across this boundary. There are five dominant classes of systems: