DEFINED as a quantity of MATTER or a region in a space upon which attention is concentrated in the analysis of a problem.There are three classes of SYSTEMS:Closed systemOpen systemIsolated systemClosed system: The system of fixed mass is CALLED the closed system. There is no mass transfer across the system boundary. The energy transfer into or out of the system may happen.The arrangement of piston cylinder with certain quantity of FLUID is an example of closed system.Open system: The open system is one in which matter crosses the boundary of the system. The energy transfer into or out of the system may happen.Most of the engineering devices are open system, Air compressor, aircraft engine, turbine, pump, etc. are example of open system.Isolated system: The isolated system is one in which there is no interaction between the system and surroundings. It is of fixed mass and energy, and there is no mass or energy transfer across the system boundary.A thermo flask and Universe is the example of an isolated system.