Plain English Meaning
A system that sends out radio signals, listens for the echoes that bounce back off aircraft, weather, or terrain, and uses those echoes to figure out where things are and how they are moving.
Full Definition
A device that transmits radio waves and measures the energy reflected back from objects in order to determine their position, distance, and movement. In aviation, radar is used by air traffic controllers to track aircraft, by pilots to detect weather and terrain, and by ground systems to provide surveillance and separation services.