Definition
Physical System
A physical system is a delimited part of the physical world chosen for analysis. It is characterised by a collection of states, physical properties, and possible interactions with its environment. The purpose of introducing a physical system is to describe and predict its behaviour by means of a mathematical model.
Depending on the theory under consideration, the state of a physical system may be represented in different ways. For example, in classical mechanics it is typically described by positions and momenta, whereas in quantum mechanics it is described by a state in a Hilbert space.