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 without asking for login)
init 2 -- single user with more restrictions (not commonly used)
init 3 -- same as init 2 but with networking enabled (useful for mounting network shares while fixing stuff)
init 6 -- reboot
init c -- block further logins
init q -- rescan ttys

init 4 and 5 are not commonly seen on BSD systems, but are found on linux systems (where the other init numbers usually mean different things anyway -- e.g., init 4 is multiuser and init 5 is multiuser with networking and GUI; this varies from on a linux distribution to distribution basis, though. Red Hat commonly uses init 5 to mean "start up everything, including the GUI".)