Definition
The horizontal surfaces at the rear of the airplane, consisting of the horizontal stabilizer and the elevator (or, on some aircraft, a single moving stabilator). They provide pitch stability and control by generating aerodynamic forces that raise or lower the nose of the airplane.
Plain English
The flat, wing-like surfaces sticking out sideways from the tail. They keep the nose steady in flight and let the pilot point the nose up or down.
Context Anchor
Seen in taxiing discussions when the FAA explains how wind can push on the tail and why the pilot positions the flight controls correctly while moving on the ground.
Derivation
"Horizontal" means level with the ground, as opposed to the vertical fin that stands upright. Together the horizontal and vertical tail surfaces make up the empennage (the tail assembly).
Why Pilots Care
During taxi in strong winds, the position of the elevator (part of the horizontal tail surfaces) matters. A quartering tailwind can lift the tail and push the nose down, so correct control inputs protect the airplane from being upset by the wind.
Intuition Check
Do not read this as every flat surface near the tail. In this FAA context, it means the rear, mostly level flight surfaces that affect nose-up and nose-down control.
Example Sentence 1
When taxiing with a quartering tailwind, the pilot held the control wheel forward to keep the wind from getting under the horizontal tail surfaces and lifting the tail.
Example Sentence 2
A strong quartering tailwind can lift the horizontal tail surfaces during taxi, so the pilot held the elevator full forward.