Plain English Meaning
A special kind of aileron shape that sticks its nose down into the air when it moves up. That extra bit of drag helps the airplane turn cleanly instead of swinging the nose the wrong way during a roll.
Full Definition
An aileron designed with its hinge line set back from the leading edge so that, when the aileron deflects upward, the leading edge projects below the lower wing surface into the airflow. This projection increases drag on the down-going wing, helping to counteract adverse yaw produced during a roll.
Context Anchor
Encountered in discussions of airplane control surfaces, roll control, and how designers reduce adverse yaw.