Definition
Rotation of the aircraft about its longitudinal axis (the line running from nose to tail), causing one wing to move down while the other moves up. Bank refers to the resulting angle of the wings relative to the horizon; roll refers to the motion that produces it.
Plain English
The aircraft tipping sideways so one wing is lower than the other. Roll is the movement; bank is the angle the wings end up at.
Context Anchor
Seen when discussing aircraft attitude, instrument flying, and spatial disorientation, especially when the pilot cannot rely on the outside horizon.
Derivation
Bank' comes from an old word for a slope or tilted surface — the wings 'banked' like a tilted bench. 'Roll' is borrowed from the everyday sense of turning over sideways, as a log rolls. Together they describe the same sideways tipping motion of the aircraft.
Why Pilots Care
Unrecognized bank or roll creates false sensations of level flight or turning, a leading factor in loss-of-control accidents in instrument conditions.
Grounding Statement
Picture the wings no longer level: one wing drops, the other rises, and the airplane is banked or rolling.
Intuition Check
Bank does not mean a financial bank here. Roll does not mean moving forward like a wheel on the ground. In this context, both refer to the airplane tilting sideways around its nose-to-tail line.
Example Sentence 1
The pilot established a 30-degree bank to begin a standard-rate turn to the south.
Example Sentence 2
During the spatial disorientation demo, the instructor applied roll to simulate an unexpected wing drop.