Definition
The tendency of an aircraft to rotate about its vertical axis (nose left or right) as a result of asymmetric forces, most commonly produced by changes in engine power, propeller torque reaction, slipstream rotation, P-factor, or gyroscopic precession.
Plain English
The way the nose of the aircraft tries to swing left or right on its own, especially when you change power. It is sideways movement of the nose, not rolling or pitching.
Context Anchor
Seen in instrument flying when adjusting power during straight-and-level flight while trying to hold heading and keep the airplane coordinated.
Derivation
Yaw' is an old nautical term meaning to deviate from a straight course, used for ships swinging off heading before being adopted in aviation for the same motion about the vertical axis.
Why Pilots Care
Uncorrected yawing effect causes heading deviations, uncoordinated flight, and added workload in instrument conditions.
Grounding Statement
Picture adding power and seeing the nose start to swing slightly to one side even though you wanted to keep flying straight.
Intuition Check
Do not confuse yawing with banking or turning by itself. Yawing is the nose swinging left or right; the pilot uses rudder to control that swing.
Example Sentence 1
When the pilot advanced the throttle for the climb, the yawing effect pulled the nose left until right rudder was applied.
Example Sentence 2
Reducing power in straight-and-level flight lessened the yawing effect and helped hold heading.