change of a quantity
The increment of the value of a quantity Q with time. The change may be expressed either infinitesimally at time t by the differential dQ or dQ(t), or in practice it may be expressed by a finite increment over the time interval t1;t2, i.e. Q(t2) - Q(t1), which may be written Δ Q or ΔQ(t1;t2) \[\Delta Q = \Delta Q(t_{1};t_{2}) = Q(t_{2})- Q(t_{1})\] Examples are: mass change, Δm; amount of substance change, Δn; volume change, ΔV; substance concentration change, Δc.
PAC, 1992, 64, 1569. 'Quantities and units for metabolic processes as a function of time (IUPAC Recommendations 1992)' on page 1571 (