a device that sends out radio waves for finding out the position and speed of a moving object (such as an airplane)
a powerful radar
The approaching planes were detected by radar.
— 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.