Plain English Meaning
A small device with adjustment screws that fine-tunes the compass system to cancel out magnetic interference from the aircraft itself, so the heading shown is accurate.
Full Definition
A small adjustment device in a remote indicating compass system that uses two sets of corrective magnets, controlled by N-S and E-W adjusting screws, to cancel out magnetic deviation errors caused by the aircraft's own electrical and metallic components. It is mounted near the flux valve (the compass system's magnetic sensor) and is set during a compass swing to bring heading errors within acceptable limits on each cardinal heading.