Definition
Newton's Third Law of Motion, which states that for every action there is an equal and opposite reaction. In aviation, it explains how a force exerted in one direction produces an equal force in the opposite direction, and is one of the fundamental principles behind how aircraft generate thrust and lift.
Plain English
If you push on something, it pushes back on you with the same strength in the opposite direction. Every force has a matching force going the other way.
Context Anchor
Seen in aerodynamic discussions of lift, thrust, propellers, jet engines, and how an airplane interacts with the air around it.
Derivation
From Latin re- meaning 'back' and actio meaning 'a doing or acting.' A 'reaction' is literally an 'acting back.' That's exactly what the law describes — when you act on something, it acts back on you.
Why Pilots Care
This law is the direct reason wings generate lift and engines generate thrust, so it underpins every aspect of powered flight and control.
Analogy
If you push against a wall, the wall pushes back against you. The wall does not move much, but you can feel the force coming back through your hands.
Grounding Statement
Step off a small boat onto a dock — the boat slides backward as you step forward. Your push on the boat and the boat's push on you are equal and opposite. That is the Law of Reaction in action.
Intuition Check
Do not read “Law” here as a regulation, and do not read “Reaction” as a pilot's response time. Here it means a physical rule: a push in one direction creates an equal push back in the opposite direction.
Example Sentence 1
The Law of Reaction explains why a propeller pulling air rearward produces forward thrust on the aircraft.
Example Sentence 2
A propeller accelerates air backward, and the law of reaction drives the airplane forward.