Homoeopathy describes the treatment of a disease with something that produces the same symptoms in a healthy body. Like is cured by like. For instance, everyone has experienced the burning eyes and copious nasal discharge when cutting up raw onions. Allium cepa, made from the onion, is an excellent remedy for those symptoms, often  found in hay fever sufferers.

This Law of Similars stimulates the body’s immune system to cure a disease rather than merely suppress symptoms.
The word is derived from the Greek homoios (like) and pathos  (disease).