What is a induced drag ?
In an airfoil, there is a two-dimensional flow. In a wing, there is three-dimensional flow. In an aircraft wing lift is generated by a pressure difference, where there is a high pressure on the bottom surface and a low pressure on the top surface of the wing. This net imbalance creates lift, however, due to this pressure imbalance there is flow near the wingtip, which tends to curl, the flow of air tends to curl around tips, being forced from high-pressure region just underneath the tips to the top of the low-pressure region.
The flow gets circulatory motion and trails downstream of the wing, which is called a trailing vortex. These vortices induce a downward component of velocity, called downwash near the wing. Downwash inclines the local relative wind in the downward direction. A local lift vector is created perpendicular to the local relative wind, and there is a component of the local lift vector in the direction of the local relative wind which is a drag. This drag is called induced drag. In aerodynamics, the induced drag is also called lift-induced drag, vortex drag, or drag due to lift.