The Scoville scale is a measure of the "hotness" of a chili pepper. These fruits of the Capsicum genus contain capsaicin, a chemical compound which stimulates thermoreceptor nerve endings in the skin, especially the mucus membranes, and the number of Scoville Heat Units (SHU) indicates the amount of capsaicin present. Many hot sauces use their Scoville rating in advertising as a selling point. The scale is named after its creator, chemist Wilbur Scoville.
Scoville's original method for testing hotness was called the Scoville Organoleptic Test, which he developed in 1912. As originally devised, a solution of the pepper extract is diluted in sugar water until the "heat" is no longer detectable to a panel of (usually five) tasters; the degree of dilution gives its measure on the Scoville scale. Thus a sweet pepper, containing no capsaicin at all, has a Scoville rating of zero, meaning no heat detectable even undiluted. Conversely, the hottest chiles, such as habaneros, have a rating of 300,000 or more, indicating that their extract has to be diluted 300,000-fold before the capsaicin present is undetectable. The greatest weakness of the Scoville Organoleptic Test is its imprecision, because it relies on human subjectivity. Being a natural product, the heat can vary from pepper to pepper, so this scale is just a guide.