The amount of reactant fed per unit time to the inlet of the reactor.
Source:
PAC, 1976, 46, 71. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units - Appendix II. Definitions, Terminology and Symbols in Colloid and Surface Chemistry. Part II: Heterogeneous Catalysis' on page 82 (https://doi.org/10.1351/pac197646010071)