Allopathic medicine is term applied by homeopathists to the ordinary or traditional medical practice. Many medical dictionaries define the term allopathic medicine as the treatment of disease using conventional medical therapies, as opposed the use of alternative medical or non-conventional therapies. 
The term was coined during a contentious, 19th-century debate between practioners of homeopathic, and those they derisively referred to as "allopaths." More recently, "allopathic" has been applied to medical schools accredited by the American Medical Association (which bestow M.D. degrees), as opposed to "osteopathic" for those accredited by the American Osteopathic Association (which bestow D.O. degrees).
There is controversy surrounding the applicability of the term "allopathy." Many people use the term neutrally, simply as a name for orthodox medicine when necessary to distinguish it from other medicinal paradigms. Others have used the term allopathy in a deprecatory manner.
Medical dictionaries and general usage dictionaries also give varying accounts of the meaning of allopathy. Some dictionaries define allopathic medicine as conventional medicine. Stedman's Illustrated Medical Dictionary defines it as "[r]egular medicine, the traditional form of medical practice." The Oxford English Dictionary presents a similar application: "the present prevailing system of medicine".
Some definitions use the same extension of the term, but retain some historical connotations. In addition to the definition already presented, the Oxford English Dictionary puts forth the definition of allopathy as a "term applied by homeopathists to the ordinary or traditional medical practice, and to a certain extent in common use to distinguish it from homeopathy". The Compact Oxford English Dictionary likewise defines it as "the treatment of disease by conventional means, i.e. with drugs having effects opposite to the symptoms. Often contrasted with homeopathy."
Other sources define allopathic medicine more in accord with the meaning of its word parts, allos meaning opposite and path meaning disease. Steadman's Medical Dictionary calls it a "therapeutic system in which a disease is treated by producing a second condition that is incompatible with or antagonistic to the first." Dorland's Illustrated Medical Dictionary also defines it as a "term applied to that system of therapeutics in which diseases are treated by producing a condition incompatible with or antagonistic to the condition to be cured or alleviated. Called also heteropathy."  Tabor's Cyclopedic Medical Dictionary says it's a "system of treating disease by inducing a pathological reaction that is antagonistic to the disease being treated", and presents the application of allopathy to conventional medicine as incorrect, saying it is "erroneously used for the regular practice of medicine to differentiate it from homeopathy".  The American Heritage Medical Dictionary defines it as as: "A method of treating disease with remedies that produce effects antagonistic to those caused by the disease itself."
The term is used on websites of certain U.S. medical professional organizations. For example, the Association of American Medical Colleges (AAMC) describes the type of medicine they teach as allopathic, and the American Medical Association refers to M.D. students as allopathic medical students. Similarly, the Accreditation Council for Graduate Medical Education (ACGME) describes U.S. M.D. residencies as allopathic.
Other terms that have been proposed to describe the conventional Western medical system of practice include: conventional medicine, Western medicine, evidence-based medicine, clinical medicine, scientific medicine, regular medicine, mainstream medicine, standard medicine, orthodox medicine, and authoritarian medicine.
The term was coined by Samuel Hahnemann to differentiate homeopathic practices from conventional medicine, based on the types of treatments used.
As used by homeopaths, the term "allopathy" has always referred to a principle of curing disease by administering substances that produce the opposite effect of the disease when given to a healthy human. Hahnemann used this term to distinguish medicine as practiced in his time from his use of infinitesimally small doses of substances to treat the spiritual causes of illness.
In the essay by William Jarvis cited below, he notes that "although many modern therapies can be construed to conform to an allopathic rationale (eg, using a laxative to relieve constipation), standard medicine has never paid allegiance to an allopathic principle" and that the label "allopath" was considered highly derisive by mainstream medicine.
Whorton also discusses this historical pejorative usage:
One form of verbal warfare used in retaliation by irregulars was the word "allopathy." ....... "Allopathy" and "allopathic" were liberally employed as pejoratives by all irregular physicians of the nineteenth century, and the terms were considered highly offensive by those at whom they were directed. The generally uncomplaining acceptance of "allopathic medicine" by today's MDs is an indication of both a lack of awareness of the term's historical use and the recent thawing of relations between irregulars and allopaths.
– James C. Whorton
The Companion Encyclopedia of the History of Medicine states that "Hahnemann gave an all-embracing name to regular practice, calling it 'allopathy'. This term, however imprecise, was employed by his followers or other unorthodox movements to identify the prevailing methods as constituting nothing more than a competing 'school' of medicine, however dominant in terms of number of practitioner proponents and patients." In the nineteenth century, some pharmacies labelled their products with the terms allopathic or homeopathic.
Hahnemann used the term to refer to what he saw as a system of medicine that combats disease by using remedies that produce effects in a healthy subject that are different (hence Greek root allo- "different") from those of the disease to be treated. He claimed that his theory of homeopathy, which attempts to mimic the symptoms (hence homeo-, "the same"), was a more effective and humane alternative.
Contrary to the present usage, Hahnemann reserved the term of "allopathic" medicine to the practice of treating diseases by means of drugs inducing symptoms unrelated (i.e. neither similar nor opposite) to those of the disease. He called instead "enantiopathic" or "antipathic" the practice of treating diseases by means of drugs producing symptoms opposite to those of the patient (e.g. see Organon, VI edition, paragraphs 54-56). After Hahnemann's death the term "enantiopathy" fell in disuse and the two concepts of allopathy and enantiopathy have been more or less unified. Both, however, indicate what Hahnemann thought about contemporary conventional medicine, rather than the current ideas of his colleagues. Conventional physicians had never assumed that the therapeutic effects of drugs were necessarily related to the symptoms they caused in the healthy: e.g. James Lind in 1747 systematically tested several common substances and foods for their effect on scurvy and discovered that lemon juice was specifically active; he clearly did not select lemon juice because it caused symptoms in the healthy man, either similar or opposite to those of scurvy.
Practitioners of alternative medicine have used the term "allopathic medicine" to refer to the practice of conventional medicine in both Europe and the United States since the 19th century. In the U.S., this was also referred to as regular medicine — that is, medicine that was practiced by the regulars. The practice of "conventional" medicine in both Europe and America during the 19th century is sometimes referred to as the age of 'heroic medicine' (because of the 'heroic' measures such as bleeding and purging).