A condition of uneven lift produced by the rotor system of a helicopter when it is in forward flight.

When a helicopter is hovering in still air, the lift is uniform across its rotor disk. But when it is moving in flight, the blade traveling in the same direction as the helicopter, the advancing blade, has a speed equal to its own speed plus the speed of the helicopter. The blade on the other side, the retreating blade, has a speed equal to its own speed minus the speed of the helicopter.

Since the amount of aerodynamic lift is determined by airspeed, the difference between the airspeed over the rotor blades on the two sides of the helicopter causes the lift on the side of the advancing blade to be greater than the lift produced by the retreating blade.