What is the name of the substance in hot peppers that makes them "hot"?

Capsaicin

Jenni is Hot tonight ! Capsaicin, which makes hot peppers "hot" to the human mouth, is best neutralized by casein, the main protein found in milk. The oil is also absorbed by bread. The best thing to do if you have foolishly over-indulged is to eat a few slices of bread, slowly, and then drink milk.