Clouds

You may be thinking about the term nucleation or coalescence. Creation of liquid droplets around aerosol particles is called nucleation. The aerosol particles themselves are known as seeds, while the water that form around them are known as droplets. When referring specifically to cloud formation in the atmosphere, the nucleation process is known as coalescence and the seeds can be called cloud condensation nuclei.

These seeds can be natural, as in smaller water/ice particles or synthetic chemicals used for cloud seeding, including silver iodide and dry ice.