Definition
The additional drag produced when the horizontal tail surfaces are deflected to keep the aircraft in balanced (trimmed) flight. As an aircraft's center of lift and center of gravity shift, the tail must generate an upward or downward force to hold the nose at the desired pitch attitude, and this tail load creates extra induced drag on top of the wing's drag.
Plain English
Extra drag the aircraft makes because the tail has to push or pull to keep the nose where the pilot wants it. The harder the tail has to work to hold the airplane level, the more this extra drag costs you in speed and fuel.
Context Anchor
Seen in high-speed flight and aircraft performance discussions, especially when explaining how control-surface position and aircraft loading affect speed and efficiency.
Derivation
"Trim" in aviation means adjusting a control surface so the aircraft holds an attitude without the pilot pushing or pulling on the controls. The word comes from the nautical sense of balancing a ship's load. "Trim drag" is the drag penalty paid for keeping the aircraft in that balanced state.
Why Pilots Care
Trim drag lowers cruise efficiency and increases fuel consumption at high speeds, directly affecting range and operating costs in jets.
Intuition Check
Do not read trim here as cutting something or making it neat. In aviation, trim means adjusting the aircraft so it stays balanced with little or no control pressure. Do not read drag as only something being pulled behind the airplane. Here it means extra air resistance caused by the forces needed to hold that balance.
Example Sentence 1
As the jet accelerated through Mach 0.9, the center of lift moved aft and trim drag increased, requiring more thrust to hold cruise speed.
Example Sentence 2
Moving the center of gravity forward reduces the amount of trim drag needed to hold level flight.