https://doi.org/10.1351/goldbook.S05911

The positive square root of the sum of the squares of the deviations between the observations and the mean of the series, divided by one less than the total number in the series. The standard deviation is the positive square root of the variance, a more fundamental statistical quantity.*Source: *

Orange Book, 2^{nd} ed., p. 5 (http://media.iupac.org/publications/analytical_compendium/)

Orange Book, 2