relative retention, r

in column chromatography
The ratio of the adjusted or net retention volume (time) or retention factor of a component relative to that of a standard, obtained under identical conditions:
r = V R i ' V R st ' = V N i V N st = t R i ' t R st ' = k i k st
Depending on the relative position of the peak corresponding to the standard compound in the chromatogram, the value of r may be smaller, larger or identical to unity.
PAC, 1993, 65, 819 (Nomenclature for chromatography (IUPAC Recommendations 1993)) on page 843
Orange Book, p. 105