The ratio of the ingoing volume flow rate (dV/dt) and the culture volume (V). In continuous fermentation, a measure of a rate at which the existing medium is replaced with fresh medium, D is the reciprocal of the hydraulic retention time (HRT).
Source:
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 152 (https://doi.org/10.1351/pac199264010143)