The main software that manages your computer and runs other programs. Like the manager of a building who makes sure everything works properly.
Windows, macOS, and Android are operating systems that control how your devices work and run apps.
An operating system (OS) is not a cloud provider service; it is the core software installed on a device, server, or virtual machine. In cloud computing, you typically choose an OS image (for example, Windows Server or Ubuntu Linux) when creating a VM/instance, and the cloud provider manages the underlying hardware while you manage the OS (unless using managed services).