The Offline, also referred to as "3D" and "IRL", is a randomly-occurring phenomena. Little is known about this phenomena, it's purpose, physicality, or other aspects.
The Offline materializes as an event in which a human being will randomly be stripped away from reality and be placed into a nightmarish realm dubbed "The Offline". This realm is mostly notorious for it's hideous manifestation, absolutely nothing found in The Offline is appealing or even visually non-offensive. The objects and entities found within The Offline can be mildly disgusting to genuinely vomit-inducing.
The only way to escape The Offline is to locate glowing rectangles. These rectangles, dubbed "monitors", act as a portal back to our own reality.
A common theory is that the Offline and Outside share some connection, or in fact one of them may even exist within the other.