Capsaicin
The compound in chilli peppers that causes the sensation of burning heat.
Capsaicin is concentrated in the pale inner ribs of a chilli, where the seeds attach. It triggers a receptor in the mouth and skin that normally responds to real heat, so the brain reads it as burning. It is not a taste. Capsaicin dissolves in fat and alcohol, not in water. That is why milk or yogurt soothes the burn better than water does. Chillies are native to the Americas and reached Asia and Africa only after 1500.