Movement of a component within a system or across its boundary. It may be expressed using different kinds of quantities, e.g rates of change dQ/dt or Δ Q Δ t. Examples are: mass rate, dmB/dt or Δ m B Δ t; substance rate, dnB/dt or Δ n B Δ t.
Source:
PAC, 1992, 64, 1569. 'Quantities and units for metabolic processes as a function of time (IUPAC Recommendations 1992)' on page 1573 (https://doi.org/10.1351/pac199264101569)