An operating system is a series of small parts that eventually come together and create a "Operating system".
For example, in the wild, there is a piece of land that contains multiple types of plants, animals and minerals. All of these factors come together a s create an "Ecosystem".
Another example could be a machine or mechanical system. All of the different parts come together to further operate.
Both examples show a type of Operating system that concludes in a functional group of parts that come together as one.
Hope this helps...