Plain English Meaning
The height above the ground where the air gets cold enough to freeze water. Below it, the air is warmer than freezing; above it, the air is at or below freezing.
Full Definition
The lowest altitude in the atmosphere at which the air temperature reaches 0°C (32°F). Above this altitude, temperatures are at or below freezing, creating conditions where water in clouds or precipitation can freeze on contact with the aircraft.
Context Anchor
Seen in weather briefings, forecasts, and instrument flight planning, especially when clouds, rain, or may be present.