Noun. Mid-19th century.
[ALLO- Greek allos other, different + -PATHY from Greek pathos suffering]
MEDICINE. The treatment of disease by inducing an opposite condition (i.e in the usual way). Opposite HOMOEOPATHY.
Apparently, this is often used by homeopaths and their ilk to refer to Western medicine, biomedicine, evidence based medicine or modern medicine. I love the way the OED phrases it when defining allopathy in the treatment of disease: i.e it's the usual way.