Plain English Meaning
A small fixed line or mark on the face of a heading instrument. Whatever number on the rotating dial sits behind that line is the direction the aircraft is pointing.
Full Definition
A fixed reference mark on a directional instrument — such as a magnetic compass, heading indicator, or radio magnetic indicator — against which the rotating compass card or azimuth scale is read to determine the aircraft's heading.
Context Anchor
Seen on aircraft magnetic compasses and other heading displays, especially when reading or checking the aircraft's heading.
Derivation
From the old nautical term 'lubber,' meaning a clumsy or inexperienced sailor. The 'lubber line' was the painted line on a ship's compass bowl aligned with the , so even a novice could read the heading without the full compass card. borrowed the part and the name directly from marine .