change ratio of a quantity

A term which may be expressed infinitesimally at time $t$ by a ratio of differentials $d Q 1 t d Q 2 t$ where the kind of quantities are the same but for different components in the same system. In practice, the ratio for a finite interval is:

$Δ Q 1 t 1 t 2 Δ Q 2 t 1 t 2$

Examples are: mass change ratio, $d m 1 t d m 2 t$ ; amount of substance change ratio, $d n 1 t d n 2 t$ .

Source:

PAC, 1992, 64, 1569 (Quantities and units for metabolic processes as a function of time (IUPAC Recommendations 1992)) on page 1571