relative error

https://doi.org/10.1351/goldbook.R05266
Error of measurement divided by the true value of the measurand. Since a true value cannot be determined, in practice the conventional true value is used.
Source:
VIM [Terms] [Source]
See also:
PAC, 1994, 66, 595. (Nomenclature for the presentation of results of chemical analysis (IUPAC Recommendations 1994)) on page 601 [Terms] [Paper]