lubber line

also n.
A line or mark on a mariner's compass or cathode-ray indicator that represents the heading of a ship or aircraft.

A small vertical line that indicates the boat’s heading
On a magnetic compass or a radar scope, the boat’s heading is indicated by a vertical line known as the lubber line. Many yacht compasses have two lubber lines, one at the forward end of the bowl and the other at the after end, but only one is necessary. A line projected through the lubber lines would be precisely parallel to the boat’s fore-and-aft centerline.There may also be additional offset lubber lines marked to indicate beam bearings of objects at 90 degrees to the boat’s heading, or simply to help maintain a course when the person at the helm is standing to one side or the other of the compass.The origin of the term is obscure, though it may have been applied originally to the vertical line that allowed even a “lubber”—a big clumsy seaman—to steer a precise course by compass.See also Compass Types.

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The noun has one meaning:

Meaning #1: a fixed line on a ship's compass indicating its heading
  Synonyms: lubber line, lubber's mark, lubber's point

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