Definition
A movable flight control surface (such as an aileron, elevator, or rudder) designed so that part of its area lies forward of the hinge line, or so that mass is added ahead of the hinge, in order to reduce the force the pilot must apply to move it and to help prevent flutter. Aerodynamic balancing uses airflow on the forward portion to assist deflection; mass balancing uses added weight ahead of the hinge to prevent flutter caused by the surface's own inertia.
Plain English
A control surface built so it doesn't take as much effort to move, and so it stays steady at high speeds instead of vibrating. This is done either by shaping it so the air helps push it, or by adding weight ahead of its hinge.
Context Anchor
Seen in aircraft flight-control design, maintenance inspections, and repair work on movable surfaces such as elevators, rudders, and ailerons.
Derivation
"Balance" comes from the Latin bilanx, meaning "having two scales." In a control surface, the idea is that forces (or weights) on either side of the hinge are evened out, so the surface doesn't fight the pilot or shake itself apart at speed.
Why Pilots Care
It reduces the force needed at the controls and helps keep the surface stable at higher speeds.
Intuition Check
Balanced does not mean the aircraft is trimmed or flying level here. It means the moving surface itself has its weight or air loads arranged so it behaves safely around its hinge.
Example Sentence 1
The ailerons on this aircraft are balanced control surfaces, so they respond easily even at cruise speeds.
Example Sentence 2
High-speed flight tests verified that the balanced elevators prevented excessive control loads.