Plain English Meaning
A small built-in set of correction magnets next to the compass. They are tweaked on the ground to push the compass needle back toward the correct heading, cancelling out the magnetic pull from the aircraft itself.
Full Definition
A set of small adjustable magnets mounted near the magnetic compass that is used to cancel out deviation errors caused by the aircraft's own electrical systems and ferrous metal components. The compensator assembly is adjusted during a compass swing so that the compass reads as accurately as possible on the cardinal headings.
Context Anchor
Seen in magnetic compass discussions, especially when learning how aircraft-caused compass error is checked and reduced.