Air flowing around the surface of a solid object applies a force on it. It doesn't matter whether the object is moving through a stationary mass of air (e.g. an aircraft flying through the air) or whether the object is stationary and the air is moving (e.g. a parked aircaft being subjected to strong winds) or whether both are moving. Lift is the component of this force that is perpendicular to the oncoming flow direction. Lift is always accompanied by a drag force, which is the component of the surface force parallel to the flow direction. 

On an aircraft, the dynamic effect of the air travelling over the wing (the aerofoil) creates a force (lift) which is perpendicular to the flightpath through the wing's centre of lift. Lift opposes the donward force of weight.



Figure 1: The relationship between Lift, Drag, Weight, and Thrust

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