1. Any of several fungi of the genus Claviceps, especially C. purpurea, that infect various cereal plants and form compact black masses of branching filaments that replace the grains of the host plant. Humans and livestock can be poisoned by ingesting ergot-infected grains.

2. The plant disease caused by such a fungus.

3. The dried sclerotia of ergot, usually obtained from rye seed and used as a source of several medicinally important alkaloids and as the basic source of lysergic acid.