Ergot: A fungus (Claviceps purpurea) that contaminates rye and wheat and that produces substances (alkaloids) called ergotamines. Ergotamines constrict blood vessels and cause the muscle of the uterus to contract. They have been much used for the treatment of migraines. They have also been used and misused to induce abortion. In excess, ergotamines can cause symptoms such as hallucinations, severe gastrointestinal upset, a type of dry gangrene, and a painful burning sensation in the limbs and extremities. Chronic ergot poisoning (ergotism) was rife during the Middle Ages due to the consumption of contaminated rye. Because of the burning pain, it was known as ignis sacer (holy fire) and ignis infernalis (hell's fire), and was one of the causes of St. Anthony's fire. A form of ergot was also the original basis for the illicit drug lysergic acid diethylamide (LSD).