Absolute Unbounded Manifold
The Absolute Unbounded Manifold (AUM) goes by many alternate names, such as the LCS, All That Is, Source, etc. It is the entire consciousness system as a whole, which means everything that is consciousness and all that it experiences. AUM, which is consciousness, is the media of reality. Whether or not something exists outside of consciousness is theoretically possible but practicably unknowable, as we are consciousness itself.
AUM evolved from primordial consciousness (AUO).[MBT Trilogy 1] For more explanation as to how this arose, see AUO Transitions Into AUM which is part of the series titled "Introduction to the MBT Model".
The term "AUM" was picked by Thomas Campbell as a bit of a joke, because it is also a sound some Buddhists make while meditating (i.e. when you connect with the LCS) and is regarded as "sacred" or "mystical".