God is Father, Son and Spirit loving each other. That's it.
Okay, that's a bit more appealing than Aristotle's definition. But hold on! One God or three Gods?
But this just looks to us like you've got three Gods and they just happen to like each other a lot.
What's the solution?
We're not tritheists because we don't say the Father, Son and Spirit are three individuals; we say they are three persons. ... An individual is something that can be divided off ... so it can stand all on its own. ... Persons need relationship; they can only be understood in terms of their relations. ...
God is just these three persons loving each other. But that is not to say there are three Gods here, because their love for each other is so essential to who they are that none of them would exist without the others. ...
And so, you see, Basil and the boys are really majoring on verses like 1 John 4:16, "God is love", because they're seeing love, which is the relationship between the persons, is the being of God. It makes up the divine unity. God is one because God is love, because the Father, Son and Spirit love each other.