Plain English Meaning

In a gentle turn, the airplane wants to level itself out. You have to hold the controls slightly into the turn to keep it banked.

Full Definition

The tendency of an aircraft in a shallow bank (typically less than about 20 degrees) to roll back toward wings-level on its own, requiring the pilot to hold a small amount of aileron pressure into the turn to maintain the bank.

Context Anchor

Seen in instrument flying when setting or correcting the bank for a standard-rate turn.

Derivation

Built from 'under-' (not enough) and 'bank' (the tilt of the wings in a turn). The name describes the airplane's tendency: if alone, it would end up under-banked — meaning less banked than the pilot wants.