Plain English Meaning
An out-of-cycle weather report for an airport, issued whenever the weather changes enough to matter to pilots, instead of waiting for the next hourly report.
Full Definition
An unscheduled aviation weather observation issued between routine hourly METAR reports when conditions at an airport change significantly. A SPECI is triggered by specific criteria such as a shift in wind direction or speed, a meaningful change in visibility, the onset or end of precipitation or thunderstorms, a change in ceiling or , or other -relevant changes. Its format is identical to a .