An individual who truly understands the horse's natural physical and psychological needs and maintains them as the highest  priority when keeping, working with, riding and training horses. This includes keeping the horse barefoot and using hoofboots, if needed, for protection versus shoes.  It also means learning to ride naturally so as not to harm the horse's body or hooves, and using only non-pain inducing tack, including riding bitless in either a halter or pain-free bitless bridle. 

Trainers who use artificial devices, pain to motivate, require shoes and agree to ride horses under age 4 are not "natural" horse trainers. My opinion and I feel very strongly about it.