ergot(n = noun.plant) claviceps purpurea - a fungus that infects various cereal plants forming compact black masses of branching filaments that replace many grains of the plant; source of medicinally important alkaloids and of lysergic acid;
A diseased condition of rye and other cereals, in which the grains become black, and often spur-shaped. It is caused by a parasitic fungus, Claviceps purpurea. [1913 Webster]
The mycelium or spawn of this fungus infecting grains of rye and wheat. It is a powerful remedial agent, and also a dangerous poison, and is used as a means of hastening childbirth, and to arrest bleeding. [1913 Webster]
A stub, like soft horn, about the size of a chestnut, situated behind and below the pastern joint. [1913 Webster]
Ergot, n.
1 a disease of rye and other cereals caused by the fungus Claviceps purpurea.
2 a this fungus. b the dried spore-containing structures of this, used as a medicine to aid childbirth.
Etymology
F f. OF argot cock's spur, from the appearance produced