The terminology for sample analyte retention in field-flow fractionation (FFF) rests on the same fundamental theoretical concepts as for chromatography [1–6]. The most basic phenomenon to start reasoning from is the migration velocity νa of the sample analyte zones. If retention occurs, which is a prerequisite for separation, these velocities will be lower than the carrier (mobile phase) velocity, the latter taken as the axial velocity averaged over the cross-sectional dimension, i.e. the cross-sectional mean carrier velocity . It is natural that quantitative treatment of retention be based on the relationship between these two velocities. For this purpose, the symbol R was introduced by Martin and Synge [1] and LeRosen [2, 3], who noted that “the term R is a fundamental quantity” [3]. It was defined as the relative rate of movement of the analyte zone quantified by