Pantheism, Len - the view that everything (Nature, the Universe, whatever) is God.
It still doesn't make sense to me, no matter what you call it.
Well, it's a concept which has had its adherents and which has also received a fair amount of stick down the ages. Schopenhauer (dead German philosopher bod) said that the real problem with it is a linguistic one. It doesn't add anything to say that the Universe/Nature is God; you're just cooking up an extra word for the same thing, in fact for things for which we already have words. Like Universe. And Nature. Why muddy the waters by saying that these things are God? I think it's fair to say that most theists - especially monotheists - think of their god in personal terms; something much like a great big person with human-like emotions, a god that likes this and doesn't like that, which judges, rewards and punishes on that basis and so forth. (The reason is of course obvious. All gods are man-made, purely human constructs made in our image, not vice versa). Doesn't matter how "sophisticated" about it you want to be, when it comes right down to it this is the way most theists seem to think and act when it comes to their god. But if God is the Universe/Nature, all that goes out of the window.
I don't think there's any real answer to this, save perhaps that - according to what I've read anyway - pantheists feel justified in using the term God to refer to Nature/the Universe because on emotional grounds they believe that only that word will convey the same feelings of awe and majesty that they see in the natural universe that traditional believers see in their personalistic god.
I don't really buy it myself, but that's how I've seen it defended. And they're not doing anybody any harm after all. (And could be doing a lot of good: many pantheists are very active in environmental and ecological matters, for obvious reasons).