I’d like to answer this question by saying “If you don’t know, I can’t explain it!”, but I am assuming that you do know, although you might not know that you know.
If you were here beside me right now, in my house in the Pyrenees, I would point out the window, at the mountains, the clouds and the forests, and say “That’s natural”. And I would hope that you get it.
In fact, pointing has a very respectable pedigree. It is what Taoists and Zen Masters often do. If you ask them any question, they will do one of three things. They will ignore you completely, and you might think them rude. Or they will throw a question back at you, sometimes in the form of a nonsense riddle (a ko-an). Or, most likely, they will just point at something, without saying anything. Hopefully, you will see what they are pointing at, and assume that the answer is to be found there.
So, to return to the question “What is natural?” For me, the answer is everything – absolutely everything on this planet and in the universe – except us! We have made ourselves the sole exceptions. It is the metaphysical equivalent of being American!
Now, I fully accept that we should be natural. After all, we are just as much part of Nature as a goldfinch or a waterfall. But we are not. We have created all manner of ways to cut ourselves off from Nature. So much so, that “getting back to Nature” is considered a mere leisure activity, rather than the supremely important enterprise it needs to be.
To round off for today, everything is natural, except us. I could point nearly everywhere and say “That’s natural”.