Plain English Meaning
Changing direction by tilting the airplane to one side so it curves through the air, while using all three controls together to keep the maneuver smooth and balanced.
Full Definition
A basic flight maneuver in which the airplane is rolled into a bank to change its heading, using coordinated aileron, rudder, and elevator inputs. Turns are categorized by bank angle as shallow (less than about 20°), medium (about 20° to 45°), and steep (more than about 45°).
Context Anchor
Seen early in flight training when a student learns the basic control movements for changing direction in the air.
Derivation
Turn comes from an old word meaning to or change direction. That helps here because an airplane turn is a change in the airplane’s path, not just a change in where the nose points.