Plain English Meaning
How far the wings are tilted away from level. A small tilt is a shallow turn; a larger tilt is a steeper turn.
Full Definition
The angle, measured in degrees, between the aircraft's lateral axis (wingtip to wingtip) and the horizon when the aircraft is rolled to one side. It describes how steeply the wings are tilted during a turn.
Context Anchor
You will see this term when learning turns, judging aircraft attitude, and discussing how steeply an airplane is banking.
Derivation
Bank' comes from old nautical and later aviation use, where it described leaning a vessel or aircraft to one side. Pairing it with 'angle' simply puts a number on how far the aircraft is leaned .