Plain English Meaning
A small knob on the front of a gyro instrument that lets the pilot lock the spinning part inside it. Locking it keeps the instrument from getting confused during rough manoeuvres, and it can also be used to reset the instrument to a known starting position.
Full Definition
A control on certain gyroscopic instruments — most commonly the attitude indicator and the directional gyro — that locks the gyro mechanism in a fixed position. Pulling or pushing the knob secures the gyro so it will not tumble during unusual attitudes, ground handling, or while spinning up. On older or simpler instruments, the caging knob is also used to manually re-erect the gyro to a level reference after it has been disturbed.
Context Anchor
Seen when discussing errors in reading or setting the attitude indicator during .