Any particle that serves as a nucleus leading to the formation of ice crystals without regard to the particular physical processes involved in the nucleation. The process is referred to as heterogeneous nucleation, as opposed to homogeneous nucleation, which depends on the formation of an ice particle large enough to grow by random motion of water molecules alone. Four processes are generally distinguished: 1) deposition (sorption; previously called sublimation), where the ice phase forms directly from water vapor; 2) condensation freezing, where the ice phase forms in a supercooled solution following growth and dilution of a cloud condensation nucleus; 3) contact freezing, where a supercooled droplet nucleates following contact of an ice nucleating aerosol; 4) immersion freezing, where the nucleating particle is completely immersed in the supercooled liquid, which nucleates with sufficient cooling. Because of this multiplicity of nucleation mechanisms it is often difficult to deduce the processes active in a given cloud. Artificially generated aerosols such as silver iodide show activity by all four mechanisms, but at different rates. For natural ice nucleating aerosols, activities in all modes do not generally occur. Observations strongly suggest that, whatever their physico-chemical nature, most natural nuclei act through a freezing process rather than by deposition.