StellarSystems
A system refers to a specific portion of the physical universe chosen for analysis, often to study the interactions within or across its boundaries. A system can be as simple as a single particle or as complex as the entire cosmos, depending on the scope of the study. How you define your system in a problem can change the math in a problem, but will not change what is physically happening in the problem. Everything outside the system is referred to as the environment or surroundings. Depending on how a system interacts with its environment, it can be classified as either an open or closed system.
Open systems can interact with their surroundings. This can be seen through a boiling pot of water. Steam releases from the pot, therefore the system interacts with the environment by losing mass. Closed systems have no interaction with the surroundings.