An operating system (OS) is the system software that you use to operate your computer. Examples of popular desktop operating systems are Windows, MacOS, and Linux. Examples of popular mobile operating systems are Android and iOS.
We've observed a new trend of using "operating system" to describe a set of practices that an individual or group might use, such as this post titled "One VC Wrote An OS for Her Life". This is not how we'll use OS in our material.
Read more about OSs: https://www.digitalocean.com/community/tutorials/what-is-an-operating-system