Caption: System, environment (AKA surroundings), and boundary.

Features:

1. A system is any part of reality chosen for study/analysis and/or modeling.

   The environment is the rest of reality. You are only interested in the environment insofar as it affects the system you are analyzing.

   Boundary between system and environment can be in space, time, and/or any other dimension.

2. A physical system is a system specialized to the physical sciences. But the term can be stretched sometimes to include the life sciences and even further.

   Physical system is often abbreviated to system.

   Context, as usual, decides on whether system in general, physical system, or some other kind of "system" is meant.

3. The division of the universe/reality into system and environment can seldom be done perfectly. But it can usually be done well enough to be extremely useful.

   The division is the procedure which makes any understanding possible when you think about it.

4. In everyday life, the use of the division is often NOT explicitly stated. In science, the division is usually explicitly stated or at least obvious.

5. Rather obviously, the division between system and environment would NOT work well if there were NOT natural places to make the division: i.e., if there were NOT natural systems

   And yes, there are natural systems which everyone knows: e.g., living organisms.

   Natural systems are usually, but NOT always, chosen as the systems of analysis.