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A closed system is a system in the state of being isolated from the environment. It is often used to refer to a theoretical scenario where perfect closure is an assumption, however in practice no system can be completely closed; there are only varying degrees of closure.

In physics, a closed system can exchange heat and work, but not matter, with its surroundings. In contrast an isolated system can exchange neither heat nor matter with the surroundings. For a simple system, with only one type of particle (atom or molecule), this amounts to a constant number of particles. However, for systems which are undergoing a chemical reaction, there may be all sorts of molecules being generated and destroyed by the reaction process. In this case, the fact that the system is closed is expressed by saying that the total number of each elemental atom is conserved, no matter what kind of molecule it may be a part of. Mathematically:

where N_j is the number of j-type molecules, a_ij is the number of atoms of element i in molecule j and b_i⁰ is the total number of atoms of element i in the system, which remains constant, since the system is closed. There will be one such equation for each different element in the system.

In quantum mechanics confusingly, a closed system is equivalent to an isolated system, and a system that can exchange energy with the surroundings is referred to as an open system^[1].

• Glossary of systems theory
• Dynamical system: Has components and/or flows that change over time.
• Isolated system: Has no interactions with an outside system, not even energy can flow into or out of an isolated system.
• Open system: Can be influenced by events outside of the actual or conceptual boundaries.
• Thermodynamic system