a system state (run state), represented by a number or letter that identifies the services and resources that are currently available to users
is a configuration of the system software that only allows certain selected processes to exist. Allowed processes are defined, for each runlevel, in the file /etc/inittab. There are eight defined runlevels: 0, 1, 2, 3, 4, 5, 6, S and switching among them can only be achieved by a privileged user by means of executing the commands init and telinit.
Linux has up to 10 runlevels (0-9) available (of which usually only the first 7 are defined). Each runlevel may start a different set of services, giving multiple different configurations in the same system. Runlevel 0 is defined as ``system halt'', runlevel 1 is defined as `` single user mode'', and runlevel 6 is defined as ``reboot system''. The remaining runlevels can, theoretically, be defined by the system administrator in any way. However most distributions provide some other predefined runlevels. For example, runlevel 2 might be defined as ``multi-user console'', and runlevel 5 as ``multi-user X-Window system''. These definitions vary considerably from distribution to distribution, so please check the documentation for your own distribution.
The operating mode of a UNIX system; for example, single-user, multi-user without networking, or multi-user with networking.