What force opposes the motion of an aircraft through the air?

Wings keep an airplane up in the air, but the four forces are what make this happen. They push a plane up, down, forward, or slow it down. The four forces are lift, thrust, drag, and weight. Thrust is a force that moves an aircraft in the direction of the motion. It is created with a propeller, jet engine, or rocket. Air is pulled in and then pushed out in an opposite direction. Drag is the force that acts opposite to the direction of motion. It tends to slow an object. Drag is caused by friction and differences in air pressure. An example is putting your hand out of a moving car window and feeling it pull back. Weight is the force caused by gravity. Lift is the force that holds an airplane in the air. The wings create most of the lift used by airplanes.

Source: NASA