Definition
A mounting arrangement in which a spinning gyro is supported in only one pivoted ring, allowing the gyro to tilt about a single axis. This restricted freedom limits the gyro to sensing motion about that one axis, which is why turn-and-slip indicators use a single-gimbal gyro to detect yaw (turn) only.
Plain English
The gyro is held in just one swiveling frame, so it can lean in only one direction. That one direction of freedom is what lets the instrument measure turning.
Context Anchor
Seen in descriptions of how the turn-and-slip indicator works inside the instrument panel.
Derivation
Gimbal comes from an old French word for a jointed link or pivoted ring. 'Single' simply means one such ring is used, rather than the two used in attitude and heading gyros.
Why Pilots Care
Knowing the gyro has only one gimbal explains why the turn-and-slip indicator shows rate of turn but cannot show pitch or bank attitude — it physically can't sense those movements.
Analogy
Think of a door hinge: the door can swing one way because the hinge allows motion around one line. A single gimbal gives the gyro a similarly limited kind of motion.
Intuition Check
Do not read single gimbal as meaning there is only one gyro instrument. Here it means the gyro is mounted so it can move about one axis only.
Example Sentence 1
The turn-and-slip indicator uses a single-gimbal gyro, so it senses yaw but ignores pitch and roll.
Example Sentence 2
During a coordinated turn the single gimbal keeps the instrument responsive only to yaw without unwanted tilting from pitch or roll.