Definition
A gyroscope whose spin axis is mounted in the horizontal plane, so its rigidity in space is referenced sideways rather than up and down. In a turn-and-slip indicator, the horizontal gyro spins on a lateral axis and is gimbaled so that yawing of the aircraft (movement about the vertical axis) tilts the gyro through precession, which moves the needle to indicate rate of turn.
Plain English
A spinning wheel inside the instrument that is mounted sideways instead of upright. Because it is mounted that way, it reacts to the airplane turning left or right and drives the turn needle.
Context Anchor
Seen in diagrams and descriptions of the turn-and-slip indicator.
Derivation
Horizontal refers to the orientation of the gyro's spin axis — lying flat, parallel to the wings — rather than standing upright. Naming the gyro by its axis orientation tells you what kind of motion it will sense: a horizontally mounted gyro responds to turning (yaw), while a vertically mounted one responds to pitch and roll.
Why Pilots Care
Enables the turn needle to display rate of yaw without requiring a vertical reference, giving the pilot immediate indication of uncoordinated flight or turning speed in instrument conditions.
Grounding Statement
Inside the instrument, the horizontal gyro is the spinning part that reacts to a turn and helps move the needle.
Intuition Check
Horizontal does not mean the airplane must be flying level. Here it describes how the gyro is mounted inside the instrument.
Example Sentence 1
The turn-and-slip indicator uses a horizontal gyro, so it senses the aircraft's rate of turn rather than its bank angle.
Example Sentence 2
During a standard-rate turn, the pilot checks that the horizontal gyro-driven needle matches the expected deflection for coordinated flight.