The standard deviation divided by the mean of the series. It can be calculated with the formula: \[s_{\text{r}}=\frac{s}{\overline{x}}\]
Sources:
Orange Book, 2nd ed., p. 5 (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 601 (https://doi.org/10.1351/pac199466030595)