What is the term for the force that opposes the motion of the aircraft?

The correct term for the force that opposes the motion of the aircraft is drag. Drag is a aerodynamic force that acts in the opposite direction to the aircraft's flight path, making it a critical factor in flight dynamics. It arises due to the interaction of the aircraft with the air as it moves.

As an aircraft flies, the air resistance it encounters increases with speed and is influenced by factors such as shape, surface texture, and altitude. This resistance must be overcome by thrust (the forward force produced by the engines) for the aircraft to accelerate or maintain its speed. Understanding drag is essential for pilots and engineers, as managing this force directly affects fuel efficiency, aircraft performance, and overall safety.