Definition
Thrust required (TR) is the amount of forward force an aircraft's powerplant must produce to exactly balance total drag at a given airspeed in steady, level flight. Because drag varies with airspeed, TR varies with airspeed: it is high at very slow speeds (induced drag dominant), reaches a minimum near the airspeed for best lift-to-drag ratio, and rises again at higher speeds (parasite drag dominant). The classic plot of TR versus airspeed is the thrust-required curve, and any thrust produced by the engine above this value is excess thrust available for climb or acceleration.
Plain English
It's how much push the engine has to make to keep the plane flying level at a chosen speed. Push less than that and the plane slows or descends; push more and the plane can climb or speed up.
Context Anchor
Seen in performance discussions and climb diagrams, especially when comparing thrust required with the thrust the airplane can actually produce.
Derivation
Required' simply means 'needed.' In this context it points to the specific amount of thrust needed to offset drag at a given speed — not the maximum the engine can make, just what the flight condition demands.
Why Pilots Care
Excess thrust (thrust available minus thrust required) determines how steeply the airplane can climb.
Intuition Check
TR does not mean the maximum thrust the engine can produce. It means the thrust the airplane needs at that moment to overcome drag and keep flying at that speed.
Example Sentence 1
At the airspeed for best angle of climb, the difference between thrust available and thrust required is at its greatest.
Example Sentence 2
As airspeed decreases below minimum drag speed, thrust required rises sharply.