Plain English Meaning
A false reading the magnetic compass shows when you speed up or slow down while flying east or west. Speeding up makes it look like you are turning toward the north; slowing down makes it look like you are turning toward the south, even though your heading has not changed.
Full Definition
An error displayed by the magnetic compass during changes in airspeed on an easterly or westerly heading. When the aircraft accelerates, the compass indicates a turn toward the north; when it decelerates, it indicates a turn toward the south. The error is caused by the compass card's center of gravity being offset from its pivot, combined with the vertical component of Earth's magnetic field acting on the card during the speed change.
Context Anchor
Seen in instrument flying when learning magnetic compass errors and when cross-checking the compass during changes in airspeed.