Plain English Meaning
A pilot's weather report that gets fast-tracked because what they saw or felt is dangerous enough that other pilots need to know right away.
Full Definition
A category of pilot report (PIREP) used to relay weather conditions considered urgent because they represent an immediate hazard to flight, such as severe or extreme turbulence, severe icing, hail, low-level wind shear, tornadoes, funnel clouds, waterspouts, volcanic ash, or any other weather phenomenon judged hazardous enough to warrant priority handling and rapid distribution to other pilots and forecasters.