Plain English Meaning
A ring inside the attitude indicator that lets the spinning gyro stay upright while the airplane rolls. As the airplane banks, this ring tilts with it, but the gyro inside stays level so the instrument can show how far the wings are tilted.
Full Definition
The outer pivoting ring inside an attitude indicator that rotates about the aircraft's longitudinal (roll) axis, allowing the gyroscope's spin axis to remain vertical while the airplane banks left or right.
Context Anchor
Seen in attitude indicator diagrams and explanations of how the gyro inside the instrument can show pitch and bank.