Figure: Characteristics of System Boundary
For thermodynamic analysis of a system, such as the air compressor shown in figure, attention is focused on a certain volume in space surrounding the system known as the control volume.
The control volume of a system bounded by a surface is called the control surface. Here the matter as well as energy crosses the control surface.
Everything external to the system is called the surroundings or environment.
The system is separated from the surroundings by a system boundary. The boundary may be either fixed or movable.