Caption: System, environment (AKA surroundings), and boundary.
Features:
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.
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.
The division is the procedure which makes any understanding possible when you think about it.
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.