The Scoville Heat Scale (SHU)
The Scoville scale is a measure of the "hotness" or pungency (spiciness or "heat") of a chili pepper and other spicy foods.
Chili peppers are fruits of the Capsicum genus and contain capsaicin, a chemical compound that stimulates thermoreceptor nerve endings in the skin, especially the mucus membranes, and the number of Scoville Heat Units (SHU) indicates the concentration of capsaicinoids. Many hot sauces makers use their Scoville rating as a selling point.
The scale is named after Wilbur Scoville, an American pharmacist known for his creation of the "Scoville Organoleptic Test", now known as the Scoville scale (SHU). 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.