Everything that 'is', or more simply, everything.
Existence is the Universal Set of everything, including itself. Hence Existence exists; Existence is contained within the set of Existence.
All Existences (scoped Existence) are isomorphic. The domain language of a scoped Existence may differ from another scoped Existence; all Existences exhibit the same characteristics.
The Entity is only able to experience Existence from it's perspective. If the Entity does not deem unknown Existence to Exist, then the Entity will express something as not existing. Expressing something as not existing imposes an implicit perspective on other entities.
Conceptual Systems have their internal representation of Existence due to scoping.
Equating Existence to God is an extension of Panpsychism, Panexperientialism, Pantheism.
This allows one to have a deity that is real by definition.
Contrary to some cultural definitions, Existence includes all thoughts & states by all entities. That means something that somebody makes up exists since it was created by an entity. This allows works of art, thoughts, feelings, temporary creations, etc. to exist without question or judgement. There is no need for physical manifestation, popular opinion, etc.
The state of being, existing, or occurring; beinghood. "Most people doubt the existence of the Loch Ness monster."
Empirical reality; the substance of the physical universe. (Dictionary of Philosophy; 1968)
Everything that 'is', or more simply, everything
Explicitly Scoping Existence
Existence can be contextually scoped, with a qualifying word. For example, Physical Existence means all physical phenomena. Conceptual Existence means all concepts. Mathematical Existence means all mathematical concepts. Story Existence means all entities within the story.