A thermodynamic system is defined as a finite quantity of matter or a prescribed state in space upon in which our attention is concentrated.
The system is surrounded by an envelope called system boundary, which may be a real physical surface or it may be an imaginary surface enclosed in some matter. The boundary may be finite or it may change in shape as well as volume. Everything with in the boundary is called system, while the region external to the boundary is called surroundings.
Different types of systems
If the mass within the system is remains constant is called closed system. In closed system there is no mass transfer between the system and surrounding, there is only energy transfer across the system boundary.
2. Open system
A system in which both energy and mass transfer takes place between system and surrounding, is called as an open system.
3. Isolated system
A system in which there is neither mass nor energy transfer across the boundary is called as an isolated system.