The Collaborative International Dictionary of English v.0.48:

supersonic \su`per*son"ic\, a.
   1. Moving at a speed greater than the speed of sound in air;
      -- this speed varies with altitude, since the speed of
      sound varies with altitude, as well as with temperature
      and humidity, being approximately 1129 feet per second at
      sea level and 984 feet per second at 16 miles altitude.
      [PJC]

   2. Capable of moving at speeds higher than the speed of
      sound; -- of aircraft.
      [PJC]

WordNet (r) 3.0 (2006):

supersonic
    adj 1: (of speed) greater than the speed of sound in a given
           medium (especially air); "a supersonic bomber flies so
           fast that it must release its bombs while the target is
           still over the horizon" [ant: sonic, subsonic,
           transonic]
    2: having frequencies above those of audible sound [syn:
       supersonic, ultrasonic]