Substitutive therapy or a therapeutic system in which a disease is treated by producing a second condition that is incompatible with, or antagonistic to the first (homeopathy): Allopathy is a method of treating a disease by introducing a condition that is intended to cause a pathologic reaction which will be antagonistic to the condition being treated.

Allopathy can also be described as a system of medicine in which a disease is treated by producing effects opposed to or incompatible with the effects of the disease process.

The system of medical practice of allopathy treats diseases by the use of remedies which produce effects different from those produced by the disease under treatment. Medical doctors are said to practice "allopathic" medicine.

The term "allopathy" was coined in 1842 by C.F.S. Hahnemann to designate the usual, or normal, practice of medicine (allopathy) as opposed to homeopathy, the system of therapy that he founded based on the concept that a disease can be treated with drugs (in minute doses) and so produce the same symptoms in healthy people as the disease itself.