"Induced drag is a 3D effect that comes from the creation of lift. Lift is created by deflecting a volume of air downwards. This means the deflected air has a small vertical velocity, called downwash. This downwash has an influence back at the wing, raising the angle of attack slightly. This then tilts the force vector, which is normal to the oncoming flow, back slightly, resulting in small component of force in the drag direction. This small component is called induced drag. There is no way to escape induced drag, but there is a way to minimize it, by tailoring the 3D lift distribution." ~ from David Lednicer