Definition of alkaloid: any of a class of nitrogenous organic compounds of plant or fungal origin which have pronounced physiological actions on humans. This is a lot of information to squeeze into a small video. Ergot is an alkaloid that is sometimes a reference to the fungus Claviceps purpurea, but ergot is also an alkaloid that can be found in many different forms and plants. It’s popularly known for the similar effects it has to LSD and it used to synthesize LSD. LSD is not found naturally.