Plain English Meaning
The bottom layer of the atmosphere where almost all weather happens. It reaches up to about 36,000 feet, and the air gets colder as you climb through it.
Full Definition
The lowest layer of Earth's atmosphere, extending from the surface up to roughly 36,000 feet at mid-latitudes (higher at the equator, lower at the poles). It contains nearly all of the planet's weather, water vapor, and clouds, and temperature generally decreases with altitude through this layer.
Context Anchor
Seen in aviation weather, cloud, wind, turbulence, icing, and aircraft performance discussions.