Irken PAK

"An Irken PAK. Irkens like Zim, Zir and Tak always use that for intelligence, weaponry, and everything else. It is essential for Irkens, because an Irken PAK is for the blood of Irkens. A beating heart for them. They're used to give Irkens life also. If they need it, then they will live longer and strong. Without it, they would die after ten minutes, if they don't wear their PAKs soon."

--Su Ji-Hoon, Coming in Peace

An Irken PAK is a backpack-like device that is grafted to the spine of every Irken upon their hatching. Serving as a secondary brain, the PAK contains all of an Irken's personality and statistics. It is never revealed what PAK stands for, or if it stands for anything at all other than a variation on "pack."

Purpose
The device contains all of the Irken's knowledge, effectively serving as a auxiliary brain, and it holds a great array of weapons and tools. It also contains the host's personality, charging cell, atmospheric processor, and a handy auto-reset that will "reactivate" an Irken if they suffer a fatal injury (Plague of Babies). The Irken host's job is encoded into the PAK's programming, and can be changed instantly by a Control Brain (which can also delete all data in the PAK; such is known as deactivation).

Importance
The PAK demonstrates to perfection the cybernetic development, and reliance, that Irkens possess. Unable to live without them, and essentially composed by them, the Irken and their PAK are essentially one and the same thing. They also demonstrate the domination of Irken society by the Empire's Control Brains, which have complete jurisdiction over the status of all PAKs, able to rewrite them, encode them, and where necessary, to delete them. It is unknown when or how Irkens became so reliant upon their PAKs for basic life support, or in what manner the PAK and other cybernetics differ modern Irkens from their pre-cybernetic ancestors.

Irkens can only survive 10 minutes without their PAK (explained fully in Ten Minutes to Doom, an Invader Zim unfinished episode); once it has been removed, a 'Lifeclock' appears in their mind's eye, counting down how much longer their body shell can last. The longer the Irken goes without it, the more incoherent and uncoordinated they become, and it seems to take a lot of willpower for them to muster up any strength or logic in their PAK-less state.

It's been revealed a few times that the PAK is sentient in a way, as it can act outside of the will of its host. In deleted footage from an Invader Zim episode Abducted, Zim falls through fire and is momentarily unconscious as he takes a nasty fall. His PAK legs activate, catching onto a ledge and carrying its limp owner to safety.