In thermodynamics, a thermodynamic system, originally called a working substance, is defined as that part of the universe that is under consideration.

A real or imaginary boundary separates the system from the rest of the universe, which is referred to as the environment or surroundings – sometimes called a reservoir. A useful classification of thermodynamic systems is based on the nature of the boundary and the quantities flowing through it, such as matter, energy, work, heat and entropy. A system can be anything. For instance, a cylinder, a solution in a test tube, a living organism or a planet etc.