Learner's definition of RADAR
: a device that sends out radio waves for finding out the position and speed of a moving object (such as an airplane)
[noncount]— often used figuratively in informal U.S. English
A career in politics wasn't even on my radar (screen) at that point. [=I wasn't thinking at all about a career in politics]
an actor who has fallen off the radar [=who has stopped being noticed or talked about]
He tried to stay under the radar [=unnoticed] as he went about his business.