A nucleation site is a place where a crystal can form from a solution, a bubble of vapor can form from a boiling liquid or a drop of liquid can condense from a vapor. Nucleation sites are also called nucleators.
One example of nucleation is water droplets condensing out of the air to form clouds. In order for the water to condense, the vapor first has to cool. This happens as air masses rise farther from the earth's surface. After the air cools, it still does not condense unless a nucleation site is available. In the atmosphere, nucleation sites are provided by small dust particles and bits of sea salt or soot. The water vapor in the air condenses into droplets on these tiny particles, and clouds form.