Also contains definitions of: critical embryo, homogeneous nucleus

An aggregate of a small number of atoms, molecules or ions. An embryo of such a size that the Gibbs energy at constant pressure and temperature is a maximum is called a critical embryo. An embryo larger than a critical embryo is called a homogeneous nucleus. Colloidal sols may be formed by condensation methods. When a condensation method is applied, molecules (or ions) are deposited on nuclei which may be of the same chemical species as the colloid (homogeneous nucleation) or different (heterogeneous nucleation).
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