From AMS Glossary
An inorganic chemical compound, AgI, that has a crystalline structure (symmetry; lattice spacing) similar to ice and a very low solubility in water, and can be easily generated as an aerosol.
It was discovered by Bernard Vonnegut in 1947 after a search of the crystallographic tables as an effective nucleating agent for supercooled water. In cloud seeding applications it is usually combined with small amounts of other materials (e.g., bromine, chlorine, copper) to enhance nucleating properties through change of lattice dimension to approach more closely that of ice.
Vonnegut, B. 1947. The nucleation of ice formation by silver iodide. J. Appl. Phys.. 18. p. 593.