A Scoville unit is a unit used to specify of the hotness of food, specifically chili peppers. In 1912 Wilbur Scoville, devised a system to determine how hot foods are. He used a panel of tasters to provide heat scores for different peppers. Although we still use Wilbur's name for the unit, the current method is much more scientific.
The sensation of heat that we experience from eating certain peppers is attributable to a chemical called capsaicin. The more capsaicin present in a pepper, the hotter it will seem. The Scoville scale spans from 0 to 16 million; a sweet bell pepper has 0 Scoville units because there is no capsaicin present and pure capsaicin crystals have 16,000,000 Scoville units.
As is evident from the table below, Scoville units vary widely from one species to the next. The hottest pepper ever grown is the Naga Jolokia from Assam, India which has a Scoville score of 855,000! There are also variations of heat from one pepper to the next within the same species; growing conditions, soil and other factors have an affect on the amount of capsaicin within a given pepper. The Scoville units listed below represent the maximum amount detectable within the item in question: