Plain English Meaning
If you push on something, it pushes back on you with the same force in the opposite direction. An aircraft moves forward because its engine throws air backward, and the air pushes the aircraft forward in return.
Full Definition
A principle of physics stating that for every action there is an equal and opposite reaction. In aviation, this law explains how thrust is produced: when a propeller or jet engine pushes a mass of air rearward, the air pushes the aircraft forward with equal force. It also underlies lift production, since wings deflect air downward and the air pushes the wing upward in return.
Context Anchor
Encountered in aerodynamics discussions, especially when explaining lift, thrust, propeller effects, and aircraft control forces.