Sleet has no internationally agreed definition but is reported in meteorological observations as a combination or mix of rain and snow.

Essentially, it is frozen precipitation that partially melts as it falls and has begun the melting process before it reaches the ground, so you will see both raindrops and snowflakes falling from the sky.

This is why forecasting snow can be so tricky, since tiny variations in temperature through the atmosphere can mean the difference between precipitation falling from the cloud as rain, or quickly turning to sleet or snow.