The Collaborative International Dictionary of English v.0.48:

radar \ra"dar\ (r[=a]"d[aum]r), n. [acronym from RAdio Detection
   And Ranging.]
   1. An electronic device designed to detect objects at a
      distance, and determine their distance from the device, by
      transmitting a pulse of radio waves and measuring the time
      required for the echo of the pulse to return to the
      emitting device; as, the radar showed a plane approaching
      rapidly. For more information of radar, see the article
      What is Radar?. [acronym]
      [PJC]

   2. The method of detecting objects, finding their distance,
      or determining their speed, by use of a radar[1] device;
      as, modern radar can detect objects as small as raindrops.
      [PJC]

WordNet (r) 3.0 (2006):

radar
    n 1: measuring instrument in which the echo of a pulse of
         microwave radiation is used to detect and locate distant
         objects [syn: radar, microwave radar, radio detection
         and ranging, radiolocation]