To me, when I am talking about "natural hair" - I simply mean hair that has not been permanently straightened using some kind of a chemical ingredient, e.g. relaxer, Brazilian blow dry etc. This does not include hair straightened using temporary methods such as heat.
I have no idea what a natural ingredient is! Natural is defined as "existing in or derived from nature; not made or caused by humankind."
If you use this definition then I think there are different degrees to which something can be natural and just calling something natural does not mean much.
For example, a carrot is natural, yes, but if you cook it, it's less natural. The girl that eats a raw carrot is giving her body more and better nutrients than the girl who cooks the carrot first. Similarly, a carrot picked fresh off a farm is more natural than one that has been bagged for a supermarket. That bag normally has life prolonging chemicals inside it.
Of course, if we are talking about cake vs. carrot then there is no debate about what is more natural. However, even with the cake everything that goes in was natural. The flour came from wheat, the sugar from corn or sugarcane etc. If we weren't sat on our butts all day that cake wouldn't be so bad to eat either; it would be burned as energy rather than clogging up our arteries!
Natural does not necessarily mean organic - so if that matters to you then clarify the organicness of a product; natural does not mean completely unrefined so check what, if any, refining a product went through before it reached you; natural does not mean locally grown.
Basically, outside of "natural hair" - I think the term natural means nothing if it's not given a context.
You need to define what natural means to you and then make sure that the people you are dealing with are using the same definition, e.g. organic, eco-friendly, free of certain ingredients or inputs, grown within the country you live etc. etc.
Finally, I want you to note that unless you have specific allergies or conditions most things that are not natural will cause you no harm whatsoever if you use them in the right amount; indeed, almost all "natural" ingredients will result in a negative reaction if you use them wrongly - even drinking too much water can kill you!
'Nuff said! I'm out!