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In science, the term “system” takes the place of the common English words “thing” or “entity.” Scientists use the term “system” for whatever part of the universe they are studying. They might be studying the entire ecology of a pond, the eye of a frog, or an atom in the eye of a frog. Each of these could be considered a system.

ecology of a pond is a system
Frog in a pond. The ecology of the pond can be considered a system. [Image source: By Thompsma – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=16392401]
frog's head
The eye of a frog can be considered a system which includes the iris, retina, and so on. [Image source: Public domain, https://en.wikipedia.org/wiki/Frog. Retrieved July 30, 2018]
One advantage of the word “system” is that it doesn’t specify whether the thing is matter, energy, or both. It doesn’t specify whether the thing is animate or inanimate. It does specify that there are inter-related parts, which together should be considered to form one thing.

proton electron in hydrogen atom
An atom with its nucleus (red) and electron (blue) can be considered a system. 
[Image source: Mets501 (Own work); https://en.wikipedia.org/wiki/Proton; CC-BY-SA-3; Retrieved Feb. 16, 2018.]












More specifically, a system is any group of components that can interact with each other and form a complex whole. More examples: a car with all its parts is a system. An electrical circuit is a system. A bowl of water is a system—there’s the bowl, including all its molecules, and the water, including all its molecules.

Even an electron can, loosely speaking, be called a “system.” Electrons don’t have components; but they do have a number of properties which inter-relate. Also, they can absorb photons, which are then incorporated into the electron. So, quantum physicists can call particles, including fundamental particles, “systems.”

Open System vs. Closed System

An open system is open to its environment. Energy and/or mass can transfer between the system and the environment. A closed system has a boundary around it that cuts off transfers of energy and/or mass between the system and the environment.

terrarium is a closed system
A terrarium creates a relatively closed system. Compare it with the open system of a pond, above. [Image source: Public Domain, https://commons.wikimedia.org/w/index.php?curid=784021]
For example, a frog and plants within a terrarium is a closed system if the terrarium is glassed in. Of course, the terrarium does not form an absolute closed system because photons from the sun can travel through the glass, giving the plants energy and warming the terrarium. Other interactions with the environment could include neutrinos from the sun, sound waves from the environment vibrating the terrarium, the croaking of the frog being emitted from the terrarium, and so on. But the terrarium would be a relatively closed system compared to the very open system of a frog in a pond out in the wild.