residence time (hydraulic retention time), \(t_{\text{r}}\)

in biotechnology
https://doi.org/10.1351/goldbook.R05310
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)