rate of nucleation

The number of nuclei formed in unit time per unit volume of the liquid phase.
Orange Book, 2nd ed., p. 84 [Terms] [Book]
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 [Terms] [Paper]