A thermodynamic system can be defined as a part of the universe upon which certain observations are made. The surroundings of a thermodynamic system are the rest of the universe (everything except the system). Therefore, the relationship between a thermodynamic system, its surroundings, and the universe can be expressed as follows: universe = system + surroundings.
A thermodynamic system can be classified as an open system, a closed system, or as an isolated system depending on its properties. If the system features the exchange of both energy and matter with its surroundings, it can be called an open system. If the system can only exchange energy with its surroundings (and cannot exchange matter with it), it can be called a closed system. An isolated system is a thermodynamic system that cannot exchange energy or matter with its surroundings.