Plain English Meaning
A short note from a pilot in the air telling everyone on the ground -- and other pilots -- what the weather is actually like up there right now.
Full Definition
A report of actual weather conditions encountered by an aircraft in flight, transmitted by the pilot to a ground facility (such as Flight Service or ATC) for relay to other pilots and weather services. It typically includes location, time, altitude, sky cover, visibility, temperature, wind, turbulence, icing, and any other significant observed.