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.
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)