Official Aviation Definition
A METAR (Meteorological Aerodrome Report) is a standardized alphanumeric format used worldwide to report routine surface weather observations at airports. Published hourly or half-hourly, it reports winds, visibility, runway visual range, present weather phenomena, cloud coverage, temperature, dew point, and altimeter settings.
In Simple Words
A short coded weather telegram published by airports every hour to tell pilots current wind, clouds, temperature, and visibility.
Real-World Flight Example
The METAR read "KJFK 281200Z 31015G25KT 10SM BKN040 18/06 A2992", indicating winds 310 at 15 knots gusting to 25 with 10 miles visibility.