Does Natural Mean Drug-free?

So what is a natural birth?  I presume that natural means drug-free I think...  But I am planning to use a TENS machine, and I think I would consider gas and air.  I wonder why I feel ok with these methods of pain relief and not others?  Dr. Gowri Motha does not advocate TENS machines as they interfere with the neural pathways but others in hypnobirthing suggest their use.  

I think maybe I'm okay with TENS and gas and air because they don't interfere with the natural process of surges/contractions and vaginal birth.  Will have to read more on this...