Definition
PREDICTIVE WIND SHEAR ALERT SYSTEM (PWS)
A self‐contained system used on board some aircraft to alert the flight crew to the presence of a potential wind shear. PWS systems typically monitor 3 miles ahead and 25 degrees left and right of the aircraft's heading at or below 1200' AGL. Departing flights may receive a wind shear alert after they start the takeoff roll and may elect to abort the takeoff. Aircraft on approach receiving an alert may elect to go around or perform a wind shear escape maneuver.
Source: FAA Pilot/Controller Glossary (verbatim from the Federal Aviation Regulations).