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 [Terms] [Book] PAC, 1994, 66, 595. (Nomenclature for the presentation of results of chemical analysis (IUPAC Recommendations 1994)) on page 599 [Terms] [Paper]