Definition
The tendency of an airplane's airframe to rotate in the opposite direction to the rotation of its propeller, in accordance with Newton's third law of motion. In a single-engine airplane with a propeller turning clockwise as viewed from the cockpit, torque reaction tries to roll the airframe to the left around its longitudinal axis.
Plain English
The propeller spins one way, so the airplane wants to roll the other way. In most U.S. single-engine airplanes the propeller turns to the right from the pilot's seat, so the airplane tries to roll to the left, and the pilot has to counter that.
Context Anchor
Seen in discussions of propeller effects, takeoff handling, and minimum control speed with one engine inoperative in a multiengine airplane.
Derivation
Torque comes from the Latin torquere, meaning 'to twist.' Reaction here is used in the Newtonian sense: for every action there is an equal and opposite reaction. The propeller is twisted one way by the engine; the airframe is twisted the other way in reaction.
Why Pilots Care
It adds to the left-turning tendencies that must be overcome with rudder and aileron after an engine failure, directly raising the speed at which directional control can be maintained.
Analogy
If you twist a tight jar lid one way, your hand and wrist feel a twist back the other way. Torque reaction is the airplane feeling that opposite twist from the propeller system.
Intuition Check
Do not read “reaction” as a delayed response by the airplane. Torque reaction is an immediate opposite twisting effect caused by the engine turning the propeller.
Example Sentence 1
On takeoff, the student felt the airplane try to roll left and learned to counter the torque reaction with right rudder and a touch of right aileron.
Example Sentence 2
During single-engine practice the instructor noted that torque reaction increases the roll toward the dead engine and must be trimmed out.