Definition
On a radar display, the unwanted return of energy reflected from objects other than the targets of interest — such as terrain, precipitation, ground vehicles, structures, birds, or atmospheric phenomena — which obscures or interferes with the controller's ability to see legitimate aircraft returns.
Plain English
Extra blips and noise on a radar screen caused by things the controller doesn't care about, like rain, hills, or buildings, which make it harder to pick out the actual aircraft.
Context Anchor
Seen in radar, air traffic control, airborne weather radar, and traffic display discussions.
Derivation
From Middle English 'clotter,' meaning to clot or form a confused mass. The everyday sense of 'a messy pile of stuff getting in the way' carries directly into the radar meaning — unwanted returns crowding the screen.
Why Pilots Care
Clutter reduces the ability to detect and track aircraft, which can affect separation, traffic advisories, and overall situational awareness in both controlled and uncontrolled airspace.
Intuition Check
Clutter does not mean loose objects in the cockpit here. It means unwanted radar information that crowds the display.
Example Sentence 1
The controller advised that radar contact was lost due to ground clutter in the area south of the field.
Example Sentence 2
Heavy rain created precipitation clutter that made it difficult to identify aircraft returns on the scope.