gunsight

From gun +‎ sight.

gunsight (plural gunsights)

1. An optical instrument which establishes an optical line or axis for the purpose of aiming a weapon.
   1. In particular, especially in allusions and comparisons, a notch.
      • 2011, Tim Mackintosh-Smith, Yemen:

        I could just see the nick, like a gunsight, in the ridge of rock through which the road passes into the coastal plain of Tihamah. The nick is called Bab al-Naqah, the Gate of the She-Camel, after an unmistakable humped rock by the road.