Physical system

[1] The collection differs from a set: all the objects must coexist and have some physical relationship.

[2] In other words, it is a portion of the physical universe chosen for analysis.

The split between system and environment is the analyst's choice, generally made to simplify the analysis.

In the study of quantum coherence, the "system" may refer to the microscopic properties of an object (e.g. the mean of a pendulum bob), while the relevant "environment" may be the internal degrees of freedom, described classically by the pendulum's thermal vibrations.

Because no quantum system is completely isolated from its surroundings,[3] it is important to develop a theoretical framework for treating these interactions in order to obtain an accurate understanding of quantum systems.