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 even in linux from a distribution to distribution basis, though. Red Hat commonly uses init 5 to mean "start up everything, including the GUI".)
It is probably worth noting that the entire "runlevel" concept is far less commonly referred to in BSD discussions than in Linux discussions. When discussing administration of a BSD system, one typically just refers to "single-user mode" and "multi-user mode", where single-user mode is one root login only (usually at the console and without a password required) and multi-user mode is normal operation of the system.
This may partly be because the concept of "GUI by default" is not very prevalent in most BSD circles the way it is in many Linux circles or non-free Unixes (Solaris, SCO, etc.); while it's certainly possible to use XDM to replace console logins with graphical login at boot time, it is neither common nor encouraged.