Init

The initial system process from which all other processes branch out. Also used (and more commonly at that) to describe what runlevel the unix system is in. It breaks down to:

init 0 -- system halt (will drop you to the Open Boot Firmware level on Sparc-based systems
init 1 -- single user with restrictions (will drop you into a root shell)
init 2 -- single user with more restrictions
init 3 -- same as init 2 but with networking enabled
init 4 -- 
init 5 -- 
init 6 -- reboot