Plain English Meaning
The dividing line in the sky between the lower part of the atmosphere, where almost all weather happens, and the calmer layer above it. Below this line, air gets colder as you climb. At and above it, that pattern stops.
Full Definition
The boundary layer between the troposphere (the lower layer of the atmosphere where weather occurs) and the stratosphere (the layer above it). Its height varies from about 28,000 feet over the poles to about 56,000 feet over the equator, and it is characterized by a sudden change in temperature lapse rate.
Context Anchor
Seen in weather and high-altitude performance discussions, especially when temperature, winds, and turbulence near the top of the weather layer are being discussed.