In thermodynamics, the system is the object under consideration. Together with the surroundings (everything not part of the system, separated from it by a real or imaginary boundary), it forms the universe. A system can be anything, for example a solution in a test tube, a living organism, or a planet.

Thermodynamics is conducted under a system-centered view of the universe. All quantities (such as pressure or mechanical work) in an equation refer to the system unless labeled otherwise. For example, the equation w = 152 J means that 152 joules of work were done on the system.

Systems are divided into three types:

• An isolated system can exchange neither energy nor matter with its surroundings.
• A closed system can exchange energy but not matter with its surroundings.
• An open system can exchange both matter and energy with its surroundings. All real systems are open, though they can sometimes be theoretically approximated as closed or isolated.

For instance, consider the system of hot liquid water and solid table salt in a sealed, insulated test tube held in a vacuum (the surroundings). The test tube constantly loses heat (in the form of black-body radiation), but the heat loss progresses very slowly. If there is another process going on in the test tube, for example the dissolution of the salt crystals, it will probably occur so quickly that any heat lost to the test tube during that time can be neglected. (Thermodynamics does not measure time, but it does sometimes accept limitations on the timeframe of a process.)

Likewise, the system loses matter to its surroundings. The materials that the test tube and insulation are made of will gradually dissolve in the air. This process, too, can usually be neglected.