n.
a system for detecting the presence, direction, and speed of aircraft, ships, etc., by sending out pulses of radio waves which are reflected off the object back to the source.
▸used to indicate that someone or something has or has not come to the attention of a person or group:
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he's off the radar in the UK but in his country of birth he's a well-known figure.
Etymology
1940s: from ra(dio) d(etection) a(nd) r(anging).
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