RADAR is an acronym for 'RAdio Detection And Ranging' or 'Remote Aerial Detection And Ranging'. It is a sensing technique used on Earth in the 20th and 21st centuries. By projecting radio waves and analyzing any reflected waves, RADAR devices can attempt to identify the heading, speed, size, altitude, distance and even shape of the object which reflected them.
One of the computer displays aboard the USS Enterprise-B during its red alert in Star Trek Generations shows a radar. The video tape for this display was sold off on the It's A Wrap! sale and auction on eBay. [1]