- A system is a collection of matter of fixed identity always the same atoms or fluid particles, which may move, flow, and interact with its surroundings.
- A control volume, is a volume in space (geometric entity, independent of mass) through which fluid may flow. It can be fixed, in motion, or deformable.
In thermodynamics, all laws are based on the system. In applications, the control volume is useful.
We also have the control surface (surface of the control volume).