Natural is birthing vaginally using nothing other than things any member of the public can get their mitts on. Birth pools, TENS, homeopathy, aromatherapy, paracetamol....that kind of shizzle. No G&A and no pharmaceuticals. I think some people say 'natural' when they mean 'vaginal' as in not c section. That's not the definition of natural. It's something that's happened in the US because the use of intervention is so common place. So they say natural or CS.
BUT as I said before call it what you like. It's birth no matter how it happens!