rate of nucleation

The number of nuclei formed in unit time per unit volume of the liquid phase.
Orange Book, 2nd ed., p. 84 (http://media.iupac.org/publications/analytical_compendium/)
See also:
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 608 (https://doi.org/10.1351/pac197231040577)