Homeopathy is a system of natural health care in which extreme dilutions of substances are used to stimulate a healing response. The basic principle of homeopathy is that substances that elicit a particular symptom picture in their physical form (for example onion causing itchy watery eyes and a runny nose with discharge that burns) can be used in a diluted form to stimulate the healing of the same set of symptoms.
Using this principle, a homeopath working with an individual that suffers from allergies (with the specific symptoms of watery itchy eyes and a runny nose with discharge that burns) might select homeopathic onion (Alium Cepa).