Plain English Meaning
A small lever or knob on a gyro instrument that, when pulled or pushed, holds the inner spinning parts still and snaps the display back to a neutral, level reading.
Full Definition
A device built into a gyroscopic instrument that locks the gyro's gimbals into a fixed, centered position so the instrument can be reset to a known reference. On an attitude indicator, activating the caging mechanism returns the horizon bar to level and stops the gyro from precessing while caged.
Context Anchor
Seen in discussions of attitude indicators, especially older or mechanical instruments that can be caged before use, during startup, or after the display has been disturbed.
Derivation
From 'cage,' meaning to confine or hold something in place. Calling it 'caging' fits the picture: the gyro's freely moving parts are temporarily locked inside a fixed position, like an animal held in a cage, until releases them.