From AMS Glossary
Any particle immersed within supercooled water, initiating the growth of an ice crystal to be compared with particles nucleating directly from the vapor phase (deposition nucleus).
Similar particles may nucleate at somewhat different temperatures (a few degrees) depending on the process. Observations of natural freezing nuclei indicate that there is normally present in the atmosphere a large variety of such particles with varying activation temperatures (temperatures at which they become effective nucleators). Certain bacteria from vegetation (pseudomonas syringae) nucleate ice at temperatures as high as -2°C; mineral particles (e.g., clays: kaolinite and montmorillonite) at -10° to -20°C; artificial nuclei (e.g., silver iodide, lead iodide, and metaldehyde), as smoke, can be found to nucleate at intermediate temperatures, i.e., -5° to -10°C. The origin, distribution, and composition of these particles is highly variable; some are composed of a mixture with a hygroscopic component that dilutes prior to nucleation of the water by the freezing nucleus.