The term runlevel refers to a mode of operation in one of the computer operating systems that implement Unix System V-style initialization
Conventionally, seven runlevels exist, numbered from zero to six; though up to ten, from zero to nine[citation needed], may be used. S is sometimes used as a synonym for one of the levels. Only one "runlevel" is executed on bootup - run levels are not executed sequentially, i.e. either runlevel 2 OR 3 OR 4 is executed, not 2 then 3 then 4.
"Runlevel" defines the state of the machine after boot. Different runlevels are typically assigned to:
single-user mode
multi-user mode without network services started
multi-user mode with network services started
system shutdown
system reboot
