The Scoville Scale measures the relative capsaicin content of chili peppers. Capsaicin is what gives peppers their heat. Sweet bell peppers rate a zero on the Scoville Scale; habanero peppers may rate as high as 350,000.
Because capsaicin levels vary due to the way a pepper is grown, Scoville ratings are generally noted as a range of Scoville Heat Units. Scoville Heat Units describe the number or sugar-water drops required to dilute the capsaicin to the point where a pepper or pepper sauce is no longer hot on the tongue.
Developed by American pharmacist William Scoville in 1912, Scoville ratings of relative heat of peppers and hot sauces were originally determined by human tasters. In 2015, liquid chromatography provides more accurate Scoville Scale results.