Foundations
Operating System
Also: OS
The software that shares a computer's processor, memory and files between programs and decides what each may touch.
In more detail
Windows, macOS and Linux are operating systems. Because every program asks the operating system for access to files, the network and hardware, it is also where most permissions are enforced. A flaw in the operating system itself is serious for the same reason: it sits underneath every protection built on top of it.