Plain English Meaning
A cloud sitting on the ground. The air has cooled enough, or picked up enough moisture, that water droplets form right where you're trying to see and fly, cutting visibility down sharply.
Full Definition
A visible mass of tiny water droplets or ice crystals suspended in the air at or near the earth's surface, reducing horizontal visibility to less than five-eighths of a statute mile (1 kilometer). Fog forms when the air near the surface is cooled to its dew point, or when sufficient moisture is added to the air to raise the dew point to the existing temperature.
Context Anchor
Pilots encounter fog in preflight weather planning, airport weather reports, forecasts, and during , , , and .