Plain English Meaning

How well the ailerons actually do their job of rolling the airplane when you move the control wheel or stick. When the wing is flying cleanly, they work well. When the wing is close to stalling, they may barely work at all.

Full Definition

The degree to which the ailerons can produce a roll response when deflected. Aileron effectiveness depends on airspeed, angle of attack, wing planform, and the airflow conditions over the outer portion of the wing where the ailerons are located. As the wing approaches a stall, the airflow over the ailerons can separate, sharply reducing or eliminating their ability to roll the aircraft.