Defined as v m, v v, and v a where the v is represent the rate of feed of the given reactant fed per unit mass, volume or surface area of the catalyst.
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)