The average time a particle or volume element of the culture resides in a bioreactor (or other device) through which a liquid medium continously flows. residence time of the liquid is the reciprocal of dilution rate.
Source:
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 164 (https://doi.org/10.1351/pac199264010143)