Lubber line

A reference on a magnetic compass and directional gyro that represents the nose of the aircraft. The heading of the aircraft is shown on the compass card opposite the lubber line.

Aviation Maintenance Technician Handbook — Airframe, p. G-21

Also defined in 1 more source

The reference line used in a magnetic compass or heading indicator.

Pilot's Handbook of Aeronautical Knowledge, p. G-18