Definition
The curved scale of marked angle increments at the top of the attitude indicator, used to read the aircraft's bank angle. Tick marks are typically shown at 10, 20, 30, 45, 60, and 90 degrees of bank, and a pointer (the bank index) moves against the scale as the aircraft rolls.
Plain English
The arc of bank-angle markings across the top of the attitude indicator. You read it to see how far the aircraft is banked left or right.
Context Anchor
Seen on an attitude indicator or electronic flight display when checking or setting the airplane’s bank during instrument flying.
Derivation
"Roll" is the aircraft's rotation about its longitudinal axis (the wing-tip-to-wing-tip motion of banking). "Scale" means a graduated set of marks for measurement. So the roll scale is simply the measurement scale for roll.
Why Pilots Care
Allows the pilot to establish and maintain a desired bank angle without outside visual references.
Analogy
It works like the marks on a ruler, but instead of measuring length, it shows how much the airplane is tilted left or right.
Intuition Check
Do not read “roll” here as forward movement along the ground. In this term, “roll” means wing tilt left or right around the airplane’s nose-to-tail line.
Example Sentence 1
Rolling into the turn, the pilot stopped the bank index at the 20-degree mark on the roll scale.
Example Sentence 2
During the approach, the roll scale helped keep the wings level while correcting for wind drift.