Open system, control volume, steady flow কী?

An open system is a fixed region of space, the control volume $\Sigma$, bounded by a surface through which fluid enters and leaves (inlet and outlet sections) and through which heat and work can be exchanged with the outside; the work received through moving parts (a shaft, a piston) other than the flow itself is the useful work $W_{\text{u}}$ (shaft work). The flow is steady when every field inside $\Sigma$ is time-independent: the mass, energy and entropy contained in $\Sigma$ are then constant.