IOS
Cisco IOS (Internetwork Operating System) is the software used in Cisco Systems routers and Cisco network switches (Earlier switches ran CatOS).
IOS is a package of routing, switching, internetworking and telecommunications functions tightly integrated with a multitasking operating system.
The IOS CLI provides a fixed set of multiple-word commands— the set available is determined by the "mode" and the privilege level of the current user. "Global configuration mode" provides commands to change the system's configuration, and "interface configuration mode" provides commands to change the configuration of a specific interface. All commands are assigned a privilege level, from 0 to 15, and can only be accessed by users with the necessary privilege. Through the CLI, the commands available to each privilege level can be defined.
Source:
https://en.wikipedia.org/wiki/Cisco_IOS