Plain English Meaning
The aircraft is tilted to one side, with one wing lower than the other, instead of flying with the wings level.
Full Definition
An aircraft orientation in which the wings are tilted relative to the horizon, producing rotation about the aircraft's longitudinal axis. The angle between the wings and the horizon is the bank angle, and the aircraft is said to be in a banked attitude any time that angle is not zero.
Context Anchor
Seen in instrument flying discussions, especially when describing how a pilot’s inner ear can wrongly sense a turn or level flight after the airplane has been in a gentle bank.