An open system is one in which

• heat and work crosses the boundary of the system, but the mass of the working substance does not crosses the boundary of the system
• mass of the working substance crosses the boundary of the system but the heat and work does not crosses the boundary of the system
• both the heat and work as well as mass of the working substance crosses the boundary of the system
• neither the heat and work nor the mass of the working substance crosses the boundary of the system