Plain English Meaning
When you start a turn from north or south, the compass briefly lies. Turning from north, it first swings the wrong way before catching up. Turning from south, it races ahead of your actual turn. These wrong readings are called lag (north) and lead (south).
Full Definition
Errors in the magnetic compass that cause it to momentarily indicate a turn in the wrong direction, or fail to indicate a turn at all, when the aircraft turns from a northerly or southerly heading. On a northerly heading, the compass initially shows a turn in the opposite direction (lag); on a southerly heading, it shows a turn faster than the actual turn (lead). These errors are caused by the vertical component of the Earth's magnetic field acting on the tilted compass card during a banked turn.
Context Anchor
Seen when studying magnetic compass errors and when practicing turns using the magnetic compass.