Why is a reduction in lift called 'drag'? Induced drag is high pressure air 'leaking' to the upper part of the wing (low pressure zone) creating a vortex which means the area of the wing tip with the vortex can't create lift. Drag on the other hand is the opposition to the flow of air. Why is induced drag called a drag when it doesn't oppose oncoming air but instead reduces wing lift?

Same thing applies to interference drag, high pressure air from the top wing of a biplane leaking to the low pressure of the wing underneath it hence reducing lift, yet it's called drag when it actually reduces lift.

So my question again is why is interference drag called a drag when in reality, it reduces lift and does not oppose oncoming air?
Induced drag is unrelated to leaking and vortex creation. It exists even in a wind tunnel where the wingtips touch the walls and thus prevent leaking and vortices.

Then what is induced drag? To create lift, the wing needs to be at some angle with respect to airflow (or be asymmetric), so the force vector points mostly upwards but also somewhat aft. Forces that point aft are generally called drag. Induced drag is thus a 'real' drag, not a reduction in lift.