Basically, it is the Chilli Heat Index.

It was in 1912 when Wilbur Scoville started testing the heat of chillies!

The Scoville Organoleptic Test was invented by pharmacist Scoville while working at Parke-Davis Pharmaceutical Company. The Scoville scale is used to determine the pungency (heat) of the chilli.

Basically, the pungency of the chilli depends on the location in which chilli is grown and the genetic structure of the chilli. The pungency of the chilli is due to capsaicin, a chemical compound found mainly in the placenta of the chilli. Capsaicin stimulates the nerve endings in the tongue and mouth which transmits to the brain making the body release endorphins which give you a feeling of well being.

Wilbur Scoville tried many different methods to measure the pungency of the chilli, but he found that the readings were not precise enough. He tried to mix the chilli extract with different chemicals but was not successful. He found that subjective tests were more successful. The tongue, being so sensitive, reacted to the pungency of the chilli. He then soaked the chilli in alcohol and found capsaicin is soluble in alcohol.

The quantity of the soaked extract is noted. Water mixed with sugar is added to the extract and given to the human subjects to taste the sample for pungency. Usually, there are five people who taste the sample. To achieve the rating, three people out of five must agree on the taste. If there is still pungency found, then the samples are further diluted with a solution of alcohol and sweetened water until there is no heat felt by the tasters.

The Scoville test is now done by High-Performance Liquid Chromatograph, a modern machine that has replaced human tasters. This machine is very sensitive to the pungency of the chilli just like the human tongue. In this method, the capsaicin is extracted from the dried and grounded chilli pods. The sample is injected into a High-Performance Liquid Chromatograph machine to detect the heat. It is less time consuming and eliminates inaccuracy.

The ratio of dilution is considered as Scoville Unit. For example, the capsicum (bell) pepper has zero in the Scoville scale, a Jalapeno around 5,000 units, habanero up to 500,000 and the Bhut and Bih Jolokia can score over 1 million.

What brought Wilbur to do this I have no idea.