The ability to perform work is said to be energy of a system. Alternatively, energy of a system can be defined as capacity of system to exert force for a certain interval. If a system has high energy content, it can provide higher magnitude of force for longer interval of time.

Exchange of energy across the boundary of a system due to the intensive property difference between system and surrounding is said to be energy transfer. Energy transfer occurs as heat transfer or work transfer.

A form of energy which constitutes total energy of a system, remains within the system boundary as inherent property of a system is said to be stored energy. Internal energy, potential energy, kinetic energy,etc., are examples of stored energy. Stored energy has unique value for each equilibrium state and is independent of path. Hence, it is a thermodynamic property.

A form of energy that can cross the boundary of a system during thermodynamic process is said to be transient energy. Heat transfer and work transfer are examples of transient energy. Transient energy is a a dynamic form of energy and do not have unique value for each equilibrium state. It depends on properties of system as well as properties of surrounding. It is dependent on path. Thus, it is not thermodynamic property.