Answer:
Air resistance
Step-by-step explanation:
Air resistance is a force due to the contact between the molecules present on the surface of an object in motion and the molecules of the air. Due to this contact, the object is slowed down in its motion, because the direction of the air resistance is always opposite to the direction of motion of the object.
In an ideal world, the motion of the objects can be analyzed neglecting air resistance: in such a case, a paper plane would continue moving without slowing down. In a real world, however, this is not true, since the air resistance acts on the plane and slows it down.