Definition
With respect to ATC clearances, traffic whose altitude, position, and intentions are known to ATC.
Plain English
Other aircraft that air traffic control can see or has been told about, so the controller knows where they are, how high they are, and what they are doing.
Context Anchor
Used in air traffic control and flight safety discussions when deciding whether another aircraft can affect a clearance, route, or maneuver.
Derivation
Known comes from an old English word meaning to recognize or understand. Traffic originally referred to movement or activity; in aviation, traffic means other aircraft operating nearby. Together, known traffic means aircraft whose movement is understood well enough to be considered by air traffic control.
Why Pilots Care
Knowing traffic is being tracked by ATC improves situational awareness and helps pilots expect possible traffic calls or vectors.
Intuition Check
Do not assume known traffic means all aircraft in the area. It only means aircraft whose position, altitude, and intentions are known to air traffic control.
Example Sentence 1
The controller cleared us through the area based on known traffic, but we kept a careful lookout for VFR aircraft that might not be talking to ATC.
Example Sentence 2
Because the traffic was known to ATC, we received a vector to avoid it while on flight following.