https://doi.org/10.1351/goldbook.T06439
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 \(\frac{\text{d}Q}{\text{d}t}\) or \(\frac{\Delta Q}{\Delta t}\). Examples are: mass rate, \(\frac{\text{d}m_{\text{B}}}{\text{d}t}\) or \(\frac{\Delta m_{\text{B}}}{\Delta t}\); substance rate, \(\frac{\text{d}n_{\text{B}}}{\text{d}t}\) or \(\frac{\Delta n_{\text{B}}}{\Delta t}\).