Definition
The angle between an airplane's lateral axis (wingtip to wingtip) and the horizon, measured in degrees, when the airplane is rolled to one side during a turn or maneuver.
Plain English
How far the wings are tilted away from level, measured in degrees. A wings-level airplane has zero bank; a steeply banked airplane has its wings tilted well off horizontal.
Context Anchor
You see this term when learning and practicing turns, especially level turns, where the airplane must be tilted to turn while holding altitude.
Derivation
Bank' comes from the old sense of a sloped or tilted surface (like a banked road or river bank). When an airplane rolls, its wings form a sloped surface relative to the horizon -- so pilots speak of the airplane being 'banked.'
Why Pilots Care
Controls the rate of turn and increases the load factor; steeper angles require more lift and raise stall speed.
Analogy
A bicycle leans into a turn. An airplane does something similar: the more it tilts, the greater the bank angle.
Intuition Check
Bank angle does not mean anything related to money or a river bank. In this context, bank means the airplane’s sideways tilt, and bank angle is the amount of that tilt in degrees.
Example Sentence 1
She rolled into a 30-degree bank angle to begin the turn toward the downwind leg.
Example Sentence 2
Increasing the bank angle tightens the turn but also raises the G-load on the airplane.