Ultimate Reality is a term used in philosophy to indicate the underlying nature of reality. It indicates the base or substrate which underlies the higher or apparent levels of what we observe to be the reality within which we exist. A quantum physicist would take the quantum level of most basic 'particles' defined within the field of Quantum Mechanics as they exist as probability fields to be the Ultimate Reality. A theologian would in general take the God or Godhead recognized within their particular religion as the Ultimate Reality. There have been religions however which have had representations of reality as for example this earth as commonly observed perched on the back of a giant turtle or perhaps more than one giant turtle, giving rise to the expression "It's turtles all the way down".
In whatever case, Ultimate Reality is taken as the base or level of reality beyond which nothing further exists. In Tom Campbell's My Big TOE, Ultimate Reality is taken as Consciousness Space or the Larger Consciousness System which can be understood as a Cellular Automaton consisting of Reality Cells which interact according to a rule set which is unknown to us.