Simply put: the divine inner essence of all things. What Andrew Cohen would call our true nature. This is what I see as being underneath the rugged exterior (i.e the persona or ego) we have created to deal with this inharmonious world!
I see "God" as the source and sum of all being, which we each are. It is the pool or source from which we draw our existence and is the origin of life and the source of it's continual replenishment!
For a description of this underlying reality, see the following article written by a scientist who experienced a revelatory expansion of consciousness.