Plain English Meaning

It is the invisible line showing the direction the engine is pushing or pulling the airplane. Where this line sits compared to the airplane's balance point changes how the nose moves up or down when you change power.

Full Definition

An imaginary line extending forward from the propeller or jet engine along the direction in which the engine produces thrust. It represents the path along which engine force acts on the airframe, and its position relative to the aircraft's center of gravity affects pitch behavior when power is added or reduced.

Context Anchor

Seen in flight control discussions when explaining why power changes can make the nose want to rise or lower, and why the elevator may be needed to hold the desired .