Plain English Meaning
Rain that turns into small ice pellets on the way down because it falls through a cold layer of air near the ground. The pellets bounce when they hit the surface.
Full Definition
Precipitation in the form of small, translucent ice pellets that form when raindrops fall through a layer of below-freezing air and freeze before reaching the ground. Sleet indicates a temperature inversion, with warmer air above a layer of colder air near the surface.
Context Anchor
Seen in warm-front weather, where warmer air can ride over cold air near the ground and cause falling precipitation to melt, then freeze again.
Derivation
From Middle English 'slete,' meaning a mix of rain and or icy precipitation. The word has carried this weather meaning for centuries, so the everyday and senses are closely related -- though aviation defines it more precisely as ice pellets, not a rain/snow mix.