The Scoville Scale is a scale of chilli strength invented in 1912 by pharmacologist Wilbur Scoville as part of his work at Parke-Davis in Detroit. Its purpose is to inform on the spicy (pseudo-heat).
Capsaicin is one of the many molecules responsible for the strength of peppers.
His method is known as the Scoville Organoleptic Test. Until 1912, there was no widely used method for measuring the heat of a pepper.
Note that the actual heat of any chile pepper can vary greatly dependent on many factors (environment, seed lineage, etc).
To establish his ranking, Wilbur Scoville prepared a solution of fresh whole chilli mashed mixed with sugar water.
This solution was generally tested by five people and as long as the burning sensation of the pepper remained, it increased the dilution.
When the burning sensation disappeared, the value of the dilution served as a measure of the strength of the pepper. For example, a mild pepper, not containing capsaicin, had a degree of zero, which means no detectable burning sensation even without dilution.
In contrast, for the strongest peppers, a rate of 300,000 meant that their extract had to be diluted 300,000 times before capsaicin became undetectable.
One of the weak points of the Scoville test was its vagueness, linked to human subjectivity, in particular because the consumption habit of chilli changes the personal level of sensitivity: a pepper can be considered as very strong by a person not used to chilli and look sweet to a person frequently consuming chilli.
This inaccuracy is also reinforced by the fact that the strength of peppers of the same variety can vary greatly because of the sun, or even the soil.
This is why liquid chromatography is now used to measure the capsaicin content of a variety of peppers.