Plain English Meaning
A short weather report made by a pilot describing what the air and sky are actually like at their altitude right now, then shared so other pilots and forecasters know what to expect.
Full Definition
A PIREP is a report of actual weather conditions encountered by a pilot in flight, transmitted to a ground facility (typically Flight Service) and disseminated to other pilots, controllers, and forecasters. PIREPs include items such as location, time, altitude, sky condition, visibility, temperature, wind, turbulence, icing, and any other significant weather observed. They supplement official forecasts and surface observations by providing real-time, in-flight weather from the only that can directly observe .