means A system that detects the location, speed, and shape of distant objects by sending out radio waves and timing the echoes that bounce back.

from An American military acronym from the early 1940s: RAdio Detection And Ranging, compressed into a tidy five-letter word. Like 'laser' and 'sonar' that followed, it slipped so smoothly into everyday speech that we forgot it was ever shouting its own job description in capital letters — and as a bonus, it reads the same backwards, a palindrome perfectly suited to a thing that listens for its own signal returning.