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]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "radar":
   AA radar, AFC, AFC mixer, CRT, CW oscillator, MAD radar, Navaglobe,
   Oboe, TR switch, TR unit, VAR, Volscan, altimeter, amplifier,
   analyzer, astronavigation, automatic electronic navigation,
   automation, avionics, cascade screen, cathode-ray tube,
   celestial navigation, chronometer, coastwise navigation, consolan,
   dead reckoning, demodulator, detector, discriminator,
   early-warning radar, electron microscopy, electron optics,
   electron physics, electronic engineering, electronic navigation,
   electronics, electrophysics, fix, frequency meter,
   gun-directing radar, hard-tube pulser, indicator,
   interception radar, limiter, line of position, local oscillator,
   long-range radar, loran, magnetron, microwave mixer, mixer,
   modulator, navar, navigation, network pulser,
   omnidirectional range, omnirange, oscillator, overlapping,
   panoramic radar, photoelectricity, pilotage, plane sailing,
   point-to-point navigation, potentiometer, pulse generator,
   radar telescope, radio, radio beacon, radio navigation, radionics,
   range-sweep amplifier, range-sweep generator, receiver, screen,
   second detector, sextant, shoran, sofar, sonar,
   square-wave generator, synchronizer, tables, tacan, teleran,
   television, timing unit, tracker, transistor physics, transmitter,
   trigatron, visual-aural range