The difference between an observed value and the arithmetic mean of the set to which it belongs. It can be calculated by the formula: \[d_{i}=x_{i} - \bar{x}\]
Sources:
Orange Book, 2nd ed., p. 4 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1994, 66, 595. 'Nomenclature for the presentation of results of chemical analysis (IUPAC Recommendations 1994)' on page 599 (https://doi.org/10.1351/pac199466030595)
See also:
VIM (https://www.iso.org/standard/45321.html)