A material either added to or present in a system, which induces either homogeneous or heterogeneous nucleation.
Source:
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)