Plain English Meaning
Tiny specks floating in the air that water vapor sticks to and turns into water droplets. They are the seeds clouds and fog grow on.
Full Definition
Microscopic particles suspended in the atmosphere -- such as dust, salt, smoke, or pollen -- onto which water vapor condenses to form cloud droplets, fog, or precipitation. Without these particles, water vapor in the air will not readily condense into visible moisture, even when the air is saturated.
Context Anchor
Seen in aviation weather discussions about cloud formation, fog, haze, and visibility.