What constitutes a ‘natural’ food? One taken from nature herself (apples, oranges, vegetables)? Or those that are harvested, packaged, and processed but still ‘natural’?

For example at work the other day someone said, oh that’s ‘natural yoghurt’. As far as I know, yoghurt isn’t ‘naturally’ occuring right? So how can it be ‘natural’? It’s made from other bits and pieces, even if you make it yourself.

I understand that twinkies aren’t ‘natural’, with all the E numbers and whatnot, but seriously what constitutes ‘natural’?