noun

• 1A person employed to assess the accuracy of the shooting on a firing range; = "marker". Now chiefly historical.

• 2An object used by a gunner as a marker to indicate the range attainable by a particular piece of artillery. Now chiefly historical.

• 3North American Nautical
  Each of two or more markers used as a navigational aid, appearing in alignment to shipping which is following a safe course.

• 4Technology
  A visual indication of distance on a radar screen.