Quackery is when someone pretends to have experience or knowledge, especially in the field of medicine. It's quackery when someone poses as a doctor.
If a person fakes being a medical doctor, that's quackery. You can also call it quackery when a company sells an herb or supplement or diet aid that doesn't actually do anything. Some doctors feel that any alternative medicine is nothing but quackery, while others believe that some of these things — like meditation or acupuncture — really work for patients. Quackery is from the 1690's, from a Dutch root word, quacksalver, "hawker of salve."