Spontaneous (irreversible) process of transfer of mass across non-homogeneous fields. The driving force can be difference in concentration (in liquids) or partial pressure (in gases) of the component. In fluids, mass transfer may be enhanced by turbulent flow. In biological systems, mass transfer through membranes may result from normal diffusion, facilitated diffusion or active transport.
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 159 (https://doi.org/10.1351/pac199264010143)